BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

VOL. 43. PP. 875-916. PL. 24. 2 FIGS.. DECEMBER 30. 1932

GONDWANA LAND BRIDGES 1

BY CHARLES SCHUCHERT

(Read before the Geological Society December 29, 1932) CONTENTS Page Introduction...... 876 Summary...... 877 Part I.—Geological evidence...... 878 On permanency...... 878 Barbados as evidence of continental marginal oscillations...... 881 History of the theory of the Gondwana land bridge...... 882 How can Gondwana Land be submerged into oceanic depths?...... 885 When did the Gondwana land bridge break down?...... 886 Conclusions...... 887 Part II.—Biogeographic evidence...... 889 Summaries of biogeographic literature...... 889 Recent and Cenozoic invertebrates of the tropical Atlantic...... 890 Living brachiopods...... 892 Living crustacea...... 893 Mesozoic ammonites...... 894 Other Mesozoic evidence...... 898 Echini...... 898 Land molluscs...... 898 Marine mammals...... 899 Land mammals of South America...... 899 Triassic land evidence in South America...... 899 Permian evidence...... 900 Devonian marine evidence...... 901 Silurian marine evidence...... 902 Part III.—Evidence from larval life...... 903 Distribution of larvae in general...... 903 Dispersal of marine invertebrates...... 904 Duration of larval life...... 906 General...... 906 Corals...... 907 Echinoderms...... 907 General...... 907 Echinoids (sea-urchins)...... 907 Starfishes...... 908 Ophiurans (brittle-stars)...... 908 Bryozoa...... 909

1 Manuscript received by the Secretary of the Society June 25, 1932. (875)

LVII— B ull. Geol. Soc. Am., Vol. 4 3 ,1 9 3 2

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Page Brachiopods . 909 Mollusca.... . 910 Crustacea... . 911 Bibliography. . . . 913

I ntroduction Nearly a century ago one school of Geology taught that every part of the land has once been beneath the sea, and every part of the oceans has once been land (Lyell). The theory of another school was: once a con­ tinent, always a continent; and its natural corollary: once an oceanic basin, always an oceanic basin (Dana). Later came an intermediate school which held that no proof exists that individual continents have always remained the same, and that parts of the ocean or even of the dry land may tomorrow sink to form new depths (Suess). The teach­ ing of the first-mentioned school has gone out of fashion, but the other two have much in common, one being strictly conservative and the other liberal in its interpretation of the permanency in the earth’s grander features. The writer belongs in the last-named school, holding that both continents and oceanic basins are, in the main, permanent features of the earth’s surface; but that they have not necessarily always had their present shape and area. The theory that land bridges once existed across the Atlantic and In­ dian oceans, postulated in the beginning by biogeographers' to explain their facts of life distribution, is popular in Europe but is not as gen­ erally accepted in America. One great difficulty with the theory is that thus far no method has been found by geologists or geophysicists to ex­ plain satisfactorily the making of such bridges, or their foundering. Having long held that the land bridges came into existence with the con­ tinents, the writer had not been concerned with their origin, but realized the difficulty of finding an explanation for their foundering. This problem formed the subject of many discussions with the late Professor Barrell, who came to the rescue of the theory by postulating that the dis­ integration of radioactive elements in the basaltic shell supplies that slow increment of heat necessary to generate new molten rocks locally, which work their way into and through the lithosphere, overloading it, so that under the principle of isostasy a mid-Atlantic bridge would sub­ side into the depths.2 This theory met with objections from geologists and geophysicists.

2 Barrell died in 1919 and among his unpublished papers was one written in 1917, en­ titled “The genesis of the earth,” a part of which was published in 1918 in “The evolution of the earth and its inhabitants” (Yale University Press). The part referred to above was published in 1927 with C. R. Longwell as editor.

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SYNTHETIC PALECGEOGRAPHIC MAP OF ALL PERMIAN TIME On Goode’s Homalographic Projection Oceans—white. Inland and shelf seas, and mediterraneans—green. Late Carboniferous in northern South America introduced to show geosynclines—green and dotted. Lands—brotvn. Land bridges and isthmian links—yellow. A, A, a probable isthmian link of Mesozoic time

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After the death of Barrell, the writer’s attention was drawn more and more to other fields, and it was not until the publication of Bailey Willis’ book, “Living Africa” (1930), with its interesting tectonic ideas, that the old question of the mechanics of foundering came back strongly to his mind. Correspondence with Willis revealed that he had distinct ideas about continental fracturing, and the writer urged him to try to apply these to the explanation of land bridges. To illustrate the fact that the Schuchert bridge across the Atlantic from South America to Africa was much narrower than that of other authors, a Permian world map traveled out to the West Coast and came back to New Haven with western Gondwana still further shrunken to a series of “isthmian links.” The discussion of which the present papers are an outgrowth was based on this map, here published as plate 24, to show to what extent our somewhat divergent, but in a measure harmonious, views have approached agreement. To accompany the map the writer has again summarized some of the already familiar biogeographic evidence, added new data, and brought together evidence to show that the larval life of marine in­ vertebrates is far too short to explain their distribution across stretches of open ocean from one continent to another. In a second paper, Pro­ fessor Willis has stated his views of the dynamic problems and attempted to show how land bridges were raised, and how they subsided.

S u m m a r y In geological literature the theory of a land bridge across the Atlantic appears to have had its origin with Marcou in 1860; but was first estab­ lished by Neumayr in 1887 and earlier, and placed in its world setting by Suess between 1885 and 1909. Neumayr called this great bridge (including South America and Africa) the Bmzil-Ethiopian continent, with the Indo-Madagascar peninsula on the east. This peninsula has since come to be known as Lemuria, the land of the lemurine mammals. Suess (1909) extended the great transverse continent to include India, and gave to the whole the name Gondwana. In the line of geologic evidence for the Gondwana bridge, the ideas of Suess as to the permanency of continents and oceans are presented in the following paper, together with what he says of Gondwana in partic­ ular; this is followed by the conclusions of W. T. Blanford (1890) and J. W. Gregory (1929). Barbados with its typical oceanic oozes is cited as evidence for the great changes that a continental margin may undergo.. The Barrell theory of continental fragmentation is briefly discussed, and a final section treats of the probable time of downbreaking of the Gondwana bridge.

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In Part II, the paleontologic evidence in the way of faunal distribution is taken up in considerable detail, along with data from the living forms that bear on Gondwana Land. Finally, the problem of how far and by what means Recent marine life is distributed is dealt with, and, in this connection, to what extent the embryonic and larval forms are dispersed and so may explain the similarities of life on either side of the tropical Atlantic. The conclusion here is that the vast majority of shallow-water life is dispersed along shorelines, and only a small percentage in any fauna is accidentally carried by the currents from one side of an ocean to the other. The conclusion of this entire study, now carried on for thirty years, is that the marine life of southern North America and southern Europe, and South America and Africa, from the Silurian to the end of Miocene time, backed by the distribution of the land plants and animals from the Permian into the Cretaceous, and by the discontinuous localization of Recent life in Africa and South America, is overwhelmingly in favor of the existence of the Gondwana land bridge from the pre-Cambrian until the end of Cretaceous time. During the submerging of the bridge, now thought by the writer to have taken place from the Eocene to the close of the Miocene, there may well have been an archipelago of islands left as remnants of transatlantic Gondwana Land, a deduction that has the backing at least of the molluscs of Miocene time and the living brachio- pods on the two tropical shores of the Atlantic Ocean. Those who lean toward the Wegener theory of continental drift as a possible explanation for the present and fossil distribution discussed in this study are referred to Hoifmann (1925), who rejects the theory as raising more difficulties than solutions in explaining the present dis­ tribution of life; and, for the paleontologic and some of the geologic difficulties, to Schuchert (1928).

P a r t I.— G e o l o g ic a l E v id e n c e

ON PERMANENCY Charles Darwin said that biogeographers were prone to construct land bridges in every convenient direction, and to sink imaginary continents in a quite reckless manner; and his contemporary, Hooker, the leading botanist of a half century ago, wrote that land bridges were the “forlorn hope of the botanical geographer.” It is still largely true that many zoologists and botanists lightheartedly extend the present continents, or erect bridges far out into the ocean, to include volcanic islands like the Bermudas or the Galapagos, or build land bridges from Florida to Ber-

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muda and Halifax, or from Central America to the volcanic mid-Pacific islands of Hawaii, or from Asia to the same islands. The problem of the permanency of the continents and oceans, or of the making and un­ making of transoceanic lands, does not concern most biogeographers. Toward the close of Suess’ lifelong and fruitful studies of the greater geological features of the earth he was asked by the editor of Natural Science to present his views on “Are great ocean depths permanent?” His answer was: “There exists no proof that individual continents always remained the same, and we even know for certain that such was not the case with by far the greater part of these continents. . . I see no reason why parts of the ocean or even of the dry land may not to-morrow sink to form new depths. From the evidence of the strata of the Himalayas and elsewhere we see that a great and deep ocean has been incorporated into the continent. . . . So I think that we must not only concede the extinction of a great Palaeozoic, Mesozoic, and Terti­ ary ocean in south-western Eurasia [=Tethys], but admit also great recent changes in the middle or southern Atlantic. Geological evidence, therefore, does not prove, nor even point to, a permanence of the great depths, at least in the oceans of the Atlantic type.”

Blanford, in a far-reaching presidential address delivered before the Geological Society of London in 1890, discusses at considerable length the evidence for and against the permanence of ocean basins. He points out that the Seychelles Islands and Madagascar of the Indian Ocean have Archean rocks, that the volcanoes of the Canary and Cape Verde islands off northwestern Africa have thrown out granite and schist, and that Ascension Island has pieces of hornblende granite. He also brings forward much biogeographic evidence in favor of for­ mer land connections and intercontinental migrations, and considers land bridges between (1) New Zealand and Australia, (2) Solomon Islands and New Guinea, (3) Africa and Madagascar, (4) Madagascar and India, and (5) South Africa and South America. He says: “No one questions for a moment that Madagascar and Africa were united during part of the Tertiary era. . . . So far as I am able to judge, every cir­ cumstance as to the distribution of life is consistent with the view that the connexion between India and South Africa included the Archaean masses of the Seychelles and Madagascar, that it continued throughout Upper Cretaceous times, and was broken up into islands at an early Tertiary date. Great depres­ sion must have taken place, and the last remnants of the islands are now doubt­ less marked by the coral atolls of the Laccadives, Maldives, and Chagos, and by the Saya de Malha bank. . . . It may be a question whether the whole of the ocean-bottom between Africa and India may not have sunk to its present depth since Cretaceous times” (1890, pages 88, 98, 99).

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Blanford believes that the evidence for the land bridge between Africa and Brazil is even stronger than that for Lemuria across the Indian Ocean. He concludes as follows: “It will be seen that whilst the general permanence of ocean-basins and con­ tinental areas cannot be said to be established on anything like firm proof, the general evidence in favour of this view is very strong. But there is no evidence whatever in favour of the extreme view accepted by some physicists and geolo­ gists that every ocean-bed now more than 1,000 fathoms deep has always been ocean, and that no part of the continental area has ever been beneath the deep sea. Not only is there clear proof that some land areas lying within continental limits have at a comparatively recent date been submerged over 1,000 fathoms, whilst sea-bottoms now over 1,000 fathoms deep must have been land in part of the Tertiary era, but there are a mass of facts both geological and biological in favour of land-connexion having formerly existed in certain cases across what are now broad and deep oceans” (page 107).

The writer has long been endorsing the principle of permanency of continents and oceans, but not at all in the rigid form propounded by Dana, because he is convinced that even though continents and oceans do not interchange their relative levels, nevertheless great parts of the pres­ ent continents have been broken down and sunk into great depths. He further holds that land bridges existed during Paleozoic and Mesozoic time in the Atlantic between Brazil and Africa, and between Africa and •India, but in the Pacific he sees no possibility for land bridges outside of Australasia and possibly in Melanesia. Therefore geologists must find a way to sink into oceanic depths such former land bridges as western Gondwana and Lemuria. Surely the Greater Antilles became separated from each other and from Central America during the Cenozoic, and in older times the East Indies from Asia and Australia, or New Caledonia and New Zealand from New Guinea, the East Indies, and Asia. On the other hand, if long tracts of former sea-bottoms like the Himalayas or the Andes have risen from 3 to 6 miles above sealevel after having previ­ ously subsided from 3 to 5 miles while being loaded with a similar thick­ ness of sediments, or if Barbados during the Miocene sank 6,000 feet and possibly 10,000 feet and then during the Pliocene rose to near sealevel where it was in late Eocene and earlier time, surely geologists in their tectonic interpretation of the earth’s crust must find a way to explain how parts of a Gondwana Land can be submerged to depths of 12,000 feet, or how an “isthmian link” like Panama or the Antilles island fes­ toon can be raised from 6,000 to 10,000 feet and studded with active volcanoes. No geologist denies that the but slightly submerged Bering bridge joined North America to Asia back at least into Triassic time

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(see Knopf, University of California, Bulletin of the Geological De­ partment, volume 5, 1910, pages 413-420), or that the Isthmus of Panama has arisen since the Jurassic; but not many of them will admit the long existence of an Atlantic land from Brazil to Africa. Matthew, in his highly interesting essay, “Climate and Evolution,” shows that he was an ardent believer in the permanency of continents and ocean basins as they now are, though his study is actually limited to Cenozoic time. He goes out of his way to say, however, that if the dis­ tribution of animals be interpreted along the lines set forth in his paper, “there is no occasion for a Gondwana Land even in the Paleozoic” (1915, page 191). In a review of his work the present writer admitted that Matthew’s conclusion was sound when restricted to the Cenozoic, “but to say there was no Gondwana in early Mesozoic time and especially none in Permian time is to drag into this painstaking and most excellent study an unnecessary and unproved conclusion.”

BARBADOS AS EVIDENCE OF CONTINENTAL MARGINAL OSCILLATIONS There can be no doubt that today on Barbados Island of the West Indies there are typical oceanic oozes, and such are known elsewhere near continental margins. Barbados is probably one of the most interesting hits of historical geology anywhere, in that the island clearly demonstrates that a continental margin (South America) can subside to at least 6,000 feet and possibly to even 10,000 feet, and later rise to 1,100 feet above sealevel. This far-reaching fact is evident from its oceanic deposits, whose unmistakable physical characters are widely accepted by oceanog­ raphers. If anyone doubts this evidence of extraordinary crustal oscil­ lations during the Oligocene and Miocene, because of the circumscribed area of Barbados, substantiating proof can be found in the clastic deposits and faunas of equivalent age in uearby Venezuela, where the Oligocene and Miocene strata are over 13,000 feet thick, and on Trinidad, where the same formations have a thickness of at least 14,000 feet. That Barbados is but a fragment of South America is shown by the fact that the oldest deposits of the island (Eocene) have several species of operculate river snails (Hemisimis) also found widely in northern South America, whence they came when Barbados was a part of Paria, the borderland of that continent. The certainty that such upheavals have taken place lends strong sup­ port to the theories of ancient transoceanic bridges or isthmian links as set forth in the next article by Willis.

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HISTORY OF THE THEORY OF THE GONDWANA LAND BRIDGE As stated earlier in this paper, the first to make a world paleogeographic map was Jules Marcou in his “Lettres sur les Eoches du Jura,” published in 1860. This map, based on the then known formations and fossils of the Jurassic, connects all of North America and much of South America with Africa and extends the latter across the Indian Ocean to embrace Peninsular India, southeastern Asia, all of the Bast Indies, and Australia. As we now see, Marcou’s lands are entirely too extensively conceived, nevertheless the conception of Gondwana Land is indicated thus early, and it was largely based, as was the land shown later by Neumayr, on the dis­ tribution of the ammonites. In 1886-1887 appeared Melchior Neumayr’s two-volume “Erdge- schichte.” In the second volume, on page 336, is his well-known world paleogeographic map of Middle Jurassic time, which is here reproduced as figure 1, because it is the basis of all subsequent maps of a similar nature. What Neumayr called the Brazil-Ethiopian continent and the Indo-Madagascar peninsula is a part of what Suess subsequently named Gondwana Land. Dana in his “Manual” (1895) tells how Oldham (1894) was struck by the absolute identity of the Permian and Triassic plants of India and South Africa, and by the similarity of the reptilian dicynodonts of the two continents. These facts and others led him to believe in a land across the Indian Ocean connecting India and South Africa throughout Mesozoic time and sinking beneath the sea in the Cenozoic era. Dana discusses this land bridge on pages 737, 873, and 937, but is not disposed to accept it. The great master of Geology, Eduard Suess, in his classic “Das Ant- litz der Erde,” published between 1885 and 1909 (English translation 1904-1909), said of Gondwana Land: “This comprises: South America from the Andes to the east coast between the Orinoco and Cape Corrientes, the Falkland islands, Africa from the south­ ern offshoots of the Great Atlas to the Cape Mountains, also Syria, Arabia, Madagascar, the Indian peninsula, and Ceylon” (IV, 1909, page 500). “We call this mass Gondwana-Land, after the ancient Gondwana flora which is com­ mon to all its parts” (I, 1904, page 596). This land, then occupied what is now the present tropical Atlantic Ocean, “first clearly discernible towards the close of the Carboniferous period, is now represented by three fragments, Africa, India and Australia” and South America. The process of breaking up con­ tinues to a comparatively late period (II, 1906, page 254). “Gondwana-Land is bounded on the north by a broad zone of marine de­ posits of Mesozoic age. This zone extends from Sumatra and Timor through Tongking and Yunnan to the Himalaya and the Pamir, Hindu Kush, and Asia

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884 CHARLES SCHUCHERT---- GONDWANA LAND BRIDGES

Minor. It must be regarded in its entirety as the relic of a sea which once extended across the existing continent of Asia. It was termed by Neumayr the ‘Central Mediterranean’, and we shall speak of it in the following pages as the Tethys. The existing Mediterranean of Europe is a remnant of the Tethys” (III, 1901, page 19). On Gondwana Land “we find representatives of the different stages of the Gondwana flora, and from India to South America we meet with reptiles which have reached a similar level of development. With the exception of certain encroachments of the upper Cretaceous, no sea has extended over this continent . . . since the Carboniferous period” (IV, 1909, page 663). “A possible continental junction [in Africa] may be sought between lats. 15° and 4° N., that is, in the bight of Biafra and to the north of it, but the possibility of its existence as far as lat. 5° S. is not excluded. . . . These facts show that it is the two parts of South America and Africa at present projecting farthest into the Ocean which afford the strongest suggestion of having been originally connected” (IV, 1909, page 666).

I t is well known that Suess held the Pacific Ocean to be of very ancient origin—the Father of Oceans—and that the lands surrounding it are framed nearly throughout by fold mountains, while the lands surround­ ing the Atlantic and Indian deeps are as a rule not so bounded and these basins attained their present forms during Cenozoic time due to the downbreaking of lands that once occupied them. He saw the appearance of modern geography with the downbreaking of the land bridges in the Indian Ocean in early Jurassic time, and those of the Atlantic in the Cretaceous. Accordingly, the high stand of all the continents toward the close of the Mesozoic when the oceans were withdrawn from all of them is but the corollary of the general deepening of all the great basins. Therefore we can say that western Gondwana and eastern Lemuria began to break down into the Atlantic and Indian oceans during the earliest Cenozoic, and that most of these lands had gone into great depths by the close of Miocene time. In the earlier Cenozoic era it may well have been that many islands and shallow-water banks, large and small, existed where western Gondwana and Lemuria had stood. The theory of Gondwana Land across the Atlantic was discussed ably and at great length by Gregory (1929) in his Presidential Address be­ fore the Geological Society of London. He has long been an upholder of this theory. The Atlantic basin, he says: “is not a fold-valley; it cuts across many folds, and there are none parallel to it. It is a long sunkland. . . . As the main subsidence of the Atlantic began in the Upper Cretaceous and was completed after the Miocene Period, it was one of the great earth-features coincident with the upbuilding of the mountains of the Alpine system and of Western America” (page cxxi).

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This address stimulated Yon Ihering to restate his well-known theory of Arehhelenis (Quarterly Journal of the Geological Society, London, volume 87,1931, pages 376-391), in a book that gives much biogeographic evidence.

HOW CAN GONDWANA LAND BE SUBMERGED INTO OCEANIC DEPTHSt Barrell answered this question as follows: “The small content of radioactive elements in the basaltic shell or astheno- sphere below the granitic crust of the continents would then supply that slow increment of heat which is necessary to generate new molten rocks. . . . Reservoirs of molten rock gather until their mass, combined with their de­ creased density in the fluid form, enables them to work their way into and through the lithosphere and demonstrate their existence in igneous activity at the surface of the earth. The magma which thus comes from the greatest depth and in greatest volume would, because of the initial density stratification, produce a notable increase in the density of the outer crust. In order to re­ establish isostatic equilibrium, such a region must subside” (1927, page 287).

As editor of this posthumous paper, Longwell says: “The proposed mechanism has been criticized on the ground that vertical intrusion can only transfer matter from one part of the lithosphere to an­ other, without altering the average density and so disturbing isostatic balance. The fallacy of this objection will be evident to the reader of this paper, as one of Barren’s fundamental postulates places the source of the great basic intrusions below the level of isostatic compensation. On this assumption the density of the intruded lithosphere is necessarily increased, and the results outlined by Barrell are in full accord with the doctrine of isostasy” (page 284).

Barrell then goes on to say that North American geologists hold strictly to Dana’s theory of the permanency of the continents and ocean basins, whereas European workers in general stand by the older view that ocean basins are broken-down portions of the granitic shell. Such broken down areas are, he says, the Red Sea, Aegean Sea, Caspian Sea, Davis Strait, Gulf of California, Death Yalley, the Rift Yalleys of eastern Africa, and Mozambique Channel. The margins of the Atlantic and Indian oceans clearly show downbreaking. Accordingly, he “accepts the European view, since, in spite of its difficulties, it yet accounts for many geological relationships. If continental fragmentation is real, it has a strong bearing upon the general problem of the origin of ocean basins” (page 288).

Willis, in the accompanying article, now presents the argument for the permanence of continents and ocean basins in a new light, recognizing the

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former existence of land bridges extending from continent to continent, but limited to the margins of oceanic deeps where now are suboceanie ridges.

WHEN DID THE GONDWANA LAND BRIDGE BREAK DOWNÎ Krenkel (1925) and Fourmarier (1926) are the leading authorities on the general geology of Africa. The former thinks that the Atlantic portion of Gondwana, which he calls Atlantis, was surely intact up to the close of the Permian, and that the same is true for Lemuria, which united southeastern Africa across Madagascar to India. The down- breaking of western Gondwana he thinks may have begun in the Permian, because of the presence of marine strata of this time in Southwest Africa, but the writer would interpret this record as the overlapping of Nereis ( = southern Atlantic) on a part of southwestern Africa. There is also a little marine Triassic in the Congo basin (Diener, 1915, calls it Congo Gulf), but farther north the transatlantic land was still intact and it is at this place that the writer begins the bridge. Then in late Jurassic and Cretaceous times (Cenomanian and Turonian) appear the overlaps of the southern Atlantic from the Gulf of Guinea south to Mossamedes in Portuguese West Africa, and finally at the end of the Cretaceous all of the bridge north to the Mediterranean appears to have been permanently separated from Africa. Along the east coast of South America the North Atlantic ( = Posei­ don) began to invade Brazil in Middle and Late Cretaceous time, so that we may assume the permanent separation to have also taken place here at the end of the Cretaceous. From the nature of the Miocene faunas in the Greater Antilles, which have so much in common with those of the Mediterranean, we may assume with Krenkel that western Gondwana was represented throughout earlier Cenozoic time by more or less widely spaced islands or banks, making it still possible for marine larval life to disperse westward into the Americas and eastward into the Mediterranean, but impossible for the larger land vertebrates to migrate across the island archipelago. This meets the objections of Matthew (1911) and other mammalogists who hold that there was no Atlantic bridge during Ceno­ zoic time. In his “Traité,” Lapparent (1906) lays the breaking up of western Gondwana toward the close of Albian time. Later there was here, he says, an island archipelago, for in Venezuela and Colombia the late Creta­ ceous faunas still have striking resemblances to those of the Mediter­ ranean. Haug’s “Traité” (1911) also favors this view. Koken (1912) breaks up Gondwana in late Neoeomian time. The paleobotanists White

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and Knowlton call for this land bridge as necessary for the dispersal of the Glossopteris flora. Off northwestern Africa are the Cape Verde and Canary islands, whose rocks show them to be continental in character and therefore parts of Africa. The Canaries have marine Upper Cretaceous, and the volcanic Madeiras farther north have Miocene strata, showing times of Atlantic overlap. On the other hand, south of the Equator and far out in the Atlantic are the Tristan da Cunha group, Saint Helena, and Ascension, all volcanic islands, but having in their agglomerates fragments of granites and schists which show their basements to be of continental rocks. Their continental connection is discussed by the late Professor E. H. L. Schwarz (1906), who sees in them evidence for the Atlantic land bridge. Daly confirms the finding of granitic pieces in the agglomerates of Ascension Island and saw one piece 2 feet across. (See his reports on Ascension (1925) and Saint Helena (1927), and that of Washington (1930) on Saint Paul’s rocks.) Lemuria, intact at least to the end of the Permian, began to submerge and separate from Africa in Lower Triassic time and was completely separated from it in the late Mesozoic. This subsiding is first seen in northwest Madagascar in the late Triassic, and the island was entirely separated in Senonian time.

CONCLUSIONS The writer’s colleague, Professor Willis, feels that the previous pages still show the influence of the Suess or continental type of transoceanic land bridge, and that the writer has not expressed the idea of isthmian links as defined by him. He says that if the land bridges are of con­ tinental character, they should be of continental rocks and structure, which has been the writer’s conception of them. Recognizing, however, the great difficulties in submerging land bridges of the dimensions drawn by Suess, Neumayr, and others, the writer has for more than ten years portrayed Gondwana as very much smaller than shown by most authors, thinking it easier to sink smaller continental-like masses than much larger ones. On the other hand, isthmian links are postulated by Willis as up- thrusted masses comparable to mountain chains, but with a tendency to­ ward a more basic composition. He asks: “Why should not a cordillera be raised in. the ocean bed, in such a position as to link together tempo­ rarily the permanent continental lands?” The writer has no objection to this postulate, since the mid-Atlantic ridge appears to him to be of the nature of a cordillera, but as to the probable internal composition of

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isthmian links, he is not well enough grounded in physical geology and geophysics to have authoritative views as to the nature of their rocks and the manner of their origin; but he is controlled by the vast abundance of facts in biogeography, and accordingly must have transoceanic land bridges to make possible the intercontinental spread of life. The prob­ lem of the making and unmaking of the bridges he leaves to the geologists. The land bridges of most biogeographers are altogether too wide, and as such should give unlimited means for intercontinental floral and faunal migrations. This, however, is just what has not happened across the trans-Atlantic bridge from northern Africa to Brazil. The best evidence here for a wide bridge is the complete spread of the Glossopteris flora of Permian time, but it could probably just as well have spread across a narrow one, the width of a mountain range and low valleys. Among the land animals, it is the invertebrates, all small forms, that are in greatest evidence (molluscs are the most conclusive), while of the large verte­ brates, such as the reptiles and the mammals, but few have ever been pointed out by biogeographers. The writer, in fact, knows of no dino­ saurs of the Jurassic or Cretaceous having used this bridge, even though large animals can spread both ways over an isthmian link, as is attested by the abundant interchange across Panama during Pliocene times, when mammals the size of elephants, horses, and camels spread into South America, and great sloths wandered into North America. Accordingly, an isthmian link with a dry land surface of 100 miles across, more or less, is wide enough to permit intercontinental exchange of plants and animals. Under the hypothesis of isthmian links, and guided by the submarine elevations between the several basins of an ocean, we are now presented with a postulate that enables us to indicate land bridges more firmly than heretofore. Surely there were other bridges than the two discussed in this paper. On the other hand, all the sunken bridges should still be more or less plainly indicated on the oceanic bottoms as highs between the ocean basins. Now that sonic sounding is coming more and more into use, we shall soon have oceanographic charts that will, with the further help of the isthmian link postulate, help us to a sound under­ standing of land bridges and biogeographic connections. Under the popular concept of the' continental type of land bridges, the writer held that the Atlantic part of Gondwana originated in the pre- Cambrian, continued as wide land throughout the Paleozoic and Meso­ zoic eras, began to subside beneath sealevel in late Mesozoic time, and was deeply submerged by the close of the Miocene. Under the

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hypothesis of isthmian links, however, this bridge is to be looked on as a far narrower one above sealevel, originating during some dynamic time, and, like land cordilleras, probably was vertically reelevated several times later on. Unlike continental cordilleras, on the other hand, the submarine part of such isthmian links can not be eroded away, but can be lowered through subsidence of the bases. The known marine biogeographic facts demand that the Brazil-Guiana link be present at least as submarine banks as early as the Silurian and long afterward, as land at least during Permian-Triassic times, making possible intercontinental migration of plant and animal life, and that it be completely submerged to great depths during the Miocene. Accordingly we may say that this link originated with the dynamic time toward the close of the Proterozoic, was accen­ tuated by the Caledonian-Acadian and the late Paleozoic unrest, and sunk by the late Mesozoic and Cenozoic crustal adjustments. Other links might be of much shorter endurance, but all should show more or less of their former presence as ridges between the deep basins of the various oceanic bottoms.

P a r t II.—B iogeographic E v id e n c e

SUMMARIES OF BIOGEOGRAPHIC LITERATURE Anyone wishing to learn what living animals and plants show in re­ gard to the Gondwana land bridge across the Atlantic will find an abundance of such evidence presented in the comprehensive two-volume work by Arldt entitled “Handbuch der Palaeogeographie” (1919-1922), and in ScharfE’s “Distribution and Origin of Life in America” (1912). This detail need not be repeated here. The evidence must necessarily be of a scattering nature, because there has been no possibility of an interchange of land plants and land animals during all of Cenozoic time between Africa and South America. Nevertheless, genetic characteristics of Mesozoic time, seen in some of the living plants and animals on the two sides of the Atlantic, are indicators of the land bridge, even though these ancient family ties are now much masked by an overgrowth of more modern features. By the biologist these masked genetic connections are fully appreciated, but it is difficult to make them understandable to the geologist. Arldt concludes: “All these numerous biogeographic facts that we have mentioned show what difficulties the relict-hypothesis presents, and that the present disper­ sion of the stocks cited is far easier to explain by the postulate of a South Atlantic land bridge [connecting Africa to Brazil], This does not mean, how­ ever, that every case cited is proof of this land bridge” (page 223).

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Scharff says that the numerous examples of the present life which he cites indicate—- “that there exists a perfectly recognizable faunistic relationship between tropical Africa and tropical South America. Nevertheless this relationship rarely extends to genera and species. . . . The faunas of Africa and South America as a whole are thoroughly distinct. . . . Consequently we must conclude with Dr. Ortmann and several other authors who have definitely expressed themselves on the geological age of the former land bridge between South America and Africa, that the latter ceased to exist before Tertiary times” (1912, pages 382-383).

Another summarizing work is “Die Geschichte des Atlantischen Oceans,” by H. von Ihering, Jena (Fischer), 1927. After the publication of the far-reaching series of volumes entitled “Biologia Centrali-Americana,” recording 38,637 species and describing 18,051 forms in 1,373 genera, the bearing of this work on the theory of the Gondwana land bridge was discussed in a symposium before the Zoological Society of London under the presidency of Professor Mac- Bride (Proceedings, 1916, volume 2, pages 541-551). Much of this vast mass of living animals bears testimony to a former land connection be­ tween South America and Africa, and the interested student is referred to this report. The distribution of insects (especially certain of the Coleoptera) and its bearing on continental connections is discussed in several recent papers by Professor Béné Jeannel, who is one of a group of zoologists contributing to the Compte Rendu of the Société de Biogéographie, founded at Paris in 1924.

RECENT AND CENOZOIC INVERTEBRATES OF THE TROPICAL ATLANTIC Many paleontologists and malacologists have remarked on the striking similarity of the West Indian-Central American faunas to those of the Mediterranean during Miocene, Oligocene, and Eocene times. Yon Iher­ ing has listed 581 species of molluscs as living along the coast of Brazil, 54 of which are known both in the Antillean islands and along the African coast, while 72 are common to the African and Brazilian seas. Dautzen- berg (1910), on the other hand, lists 352 living species of shelled molluscs off the coast of northwest Africa, and of these 15 forms in 14 genera are also found in the Antilles. This distribution, the latter thinks, must have taken place by means of larvae transported by the equatorial currents that go across the Atlantic (see, on this point, P art I I I). A later paper by Odhner (1923) reports 850 living species in the marine littoral of the West African coasts. Of these there are of endemic

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forms 540 (63 per cent); in common with the Mediterranean area 175 (20 per cent); and with the "West Indies about 62 (7 per cent). Odhner thinks this distribution is due to a transverse land bridge and to equa­ torial currents. On the other hand, 50 per cent of the land molluscs of Ascension and Saint Helena are the same as those of the West Indies, and in this case a meridional land extending southward from the trans- Atlantic bridge is hypothecated (see A, plate 24). This is also the view of Dali (1896) and Pillsbry (1905). This meridional land over the present site of the mid-Atlantic ridge, according to Odhner, has been discussed from the standpoint of geology by Jaworski (1921) and by Kober (1921), who regards the ridge as a sunken chain of mountains. According to Kiikenthal, almost nothing of the West Indian life at­ taining Bermuda gets to the Azores or the east coast of the Atlantic, since the duration of individual larval life is apparently altogether too short for it to be spread across an ocean by the most favorable currents of the present oceans. On the other hand, he points out that of the 9 species of gorgonians in the Mediterranean 4 also recur in the West Indies. The same relationship is shown by the actinians and the ascidians. This distribution, in his opinion, could have taken place only along the shore or island stations of former Gondwana (1919, page 222). The Bowden ( = late Middle .Miocene) molluscan fauna of the Antil­ lean region is known to have at least 610 species (Woodring, 1928) and of these 484 are specifically named (298 gastropods, 15 scaphopods, 171 bivalves). These species are grouped in 315 genera, most of which are still living in the West Indian waters, and of present living species there are at least 25, though it would be easy to point out 50 additional ones that are very similar to Recent kinds. The remarkable thing about this Miocene fauna in the pres'ent con­ nection is its striking generic likeness to the shell faunas of the same age in the Mediterranean region, especially the Piedmont basin of Italy. As Woodring says: “It seems unreasonable to believe that so many indistinguishable genera [30], representing many families [13], in the two regions are the result of a similar unfolding of genetically unrelated stocks. Therefore it is assumed that these genera are genetically identical and that their occurrence in both regions is the result of migration. . . . They could not have migrated around the northern border of the present Atlantic basin during Miocene time,” because this region has temperate and therefore different faunas.

Many of the tropical genera do not occur in older formations, but a few do so on one side or the other of the Atlantic. This distribution

LVIII— B ull. Geol. Soc. Am., Vol. 4 3 ,1 9 3 2

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can not be due to a free-swimming larval stage long enough to permit of crossing the tropical waters of the Atlantic unless “a series of shoal- water banks or islands extended across the southern border of the north Atlantic” (1924, page 430).

LIVING BRACHIOPODS

Blochmann holds that when living brachiopods are carefully studied they show a definite zoogeographic distribution, because after their larval metamorphoses and subsequent to their fixation these animals have no powers of movement, and all of their radiation must take place during their short larval free-floating condition. Gryphus cubensis and Terebratulina cailleti of the West Indies also occur on Ascension Island of the eastern Atlantic, a remarkable case of discontinuous distribution. Gryphus sphenoideus of the Mediterranean, according to Blochmann, is closely related to G. cubensis of the West Indies, and accordingly is indicative of the land bridge across the Atlan­ tic. He discusses these species and their distribution at some length and comes to the positive conclusion that they must have spread along the shallow waters fronting a continuous Gondwana or by means of an island archipelago composed of remnants of this land bridge (1908, page 635). Thomson leans the same way. He says:

“Oases of discontinuous distribution of the shallower-water forms have therefore a profound significance, and permit conclusions as to former land connections to be drawn” (1927, page 47).

Schuchert, in a survey of the distribution of living brachiopods—

“shows clearly not only the former existence of equatorial Gondwana across the Atlantic, but as well that its vanished Atlantic bridge still controls the distribution of living forms. We see that the genera of the northern Atlantic (Poseidon) distributed themselves in one' direction, more or less widely throughout the northern hemisphere and in another pathway eastward into the Indian Ocean by way of the northern shore of Gondwana, but the main drift was far more decidedly westward along the same land by way of the Antillean region into the Pacific, and thence in the main down the west coast of South America into the Antarctic realm” (1911, page 275).

To show the close relations between the living brachiopods of the West Indies, the Mediterranean, and the eastern Atlantic coast to the north

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and south of the Mediterranean, the following data, based on Thomson (1927), may be cited: Crania anomala, Portugal to Norway (18-808 fathoms). Genus fossil since the Ordovician. C. anomala pourtalesii, Florida and Cuba (116-226). C. lamellosa, C. Tcermes, C. rostrata, Mediterranean. Theddellina barreti, West Indies (60-163). Also Pliocene of Jamaica. Genus fossil since the Miocene. T. mediterranea testudirvaria, Miocene of Italy. Hispanirhynchia cornea, England to Canaries (240-1102). Genus fossil since the Eocene. H. eraneana, Gulf of Panama (1175). Three species fossil in Eocene and Pliocene of Italy. Qryphus cubensis, Cuba (88-2690). (?. bartletti, West Indies. Genus fossil since the Miocene or earlier. G. vitrea, Mediterranean and eastern Atlantic (40-1465). Or. sphenoideus, Portugal (200-1094). Liothyrella uva, Gulf of Tehuantepec, Peru, Magellan (18-600). Genus fos­ sil since the ?Miocene. L. affinis, Mediterranean, Azores, Florida (294-440). Also Pliocene in Italy and Algeria. L. winteri, St. Paul 1. in Atlantic (371). Dyscolia wyvillei, West Indies (390), E. Atlantic (660-1051), Indian O. (719). Genus fossil since the Pliocene. D. subquadrata, Portugal (500-600). Argyrotheca lutea, Florida and West Indies (30-127), Rio Janeiro (70). Fossil in Oligocene of Europe, Miocene of Mediterranean countries and St. Bartholomew of Lesser Antilles. A. barretiana, West Indies and Florida (43- 641), Rio Janeiro (70). A. schrammi, West Indies (25-125), A. schucherti, Miocene of Florida. A. bermudana, Bermuda. A. cordata, Mediterranean (60-100). A. cistellula, Pliocene of England, living Norway to Bay of Biscay (1-45), Sardinia, Sicily. A. cuneata, Pliocene of Italy, living in Mediterranean (9-69), Canaries (28-200). Platidia anrmioides, Miocene of Vienna, Pliocene of Italy, living in Mediter­ ranean (40-120), E. Atlantic (218-650), Florida, West Indies (88-645). Genus fossil since the Cretaceous. P. anomioides radiata, West Indies (218) and California (50-200). Pantellaria monstruosa, Pleistocene of Italy, living in Mediterranean and E. Atlantic Cape Breton to Azores (233-1389). Genus fossil since the Miocene. P. echinata, west coast Africa (331-427), Barbados (100), Florida, Cape of Good Hope (224). Dallina floridana, Florida, Gulf of Mexico, West Indies (90-310). Genus fossil since the Miocene. D. septigera, Miocene to Recent, Atlantic to Nor­ way, Canaries (20-784), Mediterranean.

LIVING CRUSTACEA Frcm the distribution of living crustaceans, Geoffrey Smith concludes: “Another connexion, at any rate during early tertiary times, which prob­ ably existed between now isolated tropical coasts, was across the Atlantic from the West Indies to the Mediterranean and West African coasts [by means of an archipelago of islands]. Numerous facts speak for this connexion. Spe­ cies of Palinurus and Dromia occur in the West Indies and the Mediterranean, which only differ from one another in detail, and a connexion between these

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two regions has been urged from the minute resemblances of the late Creta­ ceous Corals of the West Indies with those of the Gosau beds of S. Europe, and also of the Miocene land-molluscs of S. Europe with those at the present time found in the West Indies” (1920, page 202).

Smith then argues for a latitudinal arrangement of the lands to ex­ plain the distribution of the circumtropical crustacea. Dr. Mary J. Eathbun has issued in the series of U. S. National Mu­ seum bulletins three large volumes on crabs living on either side of the Americas: on the grapsoids (Bulletin 97, 1918), the spider crabs (Bulle­ tin 129, 1925), and the cancroids (Bulletin 152, 1930). Of analogous pairs of species on the two sides there are 115, and in addition there are 34 forms living in both Atlantic and Pacific oceans. These figures show that crabs, through their larval life and through their swimming ability, have ready means for wide dispersal. It is, however, not the widely dispersed forms that are most valuable in detecting land bridges, but rather the more circumscribed ones. In a letter of April 7, 1932, Doctor Eathbun writes: “There is ample evidence of a bridge between South America and Africa.”

MESOZOIC AMMONITES According to Diener (1912), most ammonites were good swimmers and were capable of raising and lowering themselves with relation to the sea- bottom, and but few, highly modified forms crawl over the bottom. Their shells are very thin, usually narrow and keeled, and some have hollow spines, all of which aid in suspension. Probably none at maturity were pelagic or open-ocean animals, and all lived in the shallow epeiric seas and along the continental shelves. After death they soon fell to the bottom, where the shells remained, as the soft body is not easily detached from the shell (because of the complicated septal flutings) as in Nautilus. All ammonite workers dwell on the remarkably wide distribution of the genera and even of the species, and their extraordinary value in chro­ nology and the establishment of faunal realms. From the Permian to the end of the Cretaceous they are probably the best guide fossils in marine invertebrate stratigraphy. The most convincing evidence in favor of western Gondwana comes from the marine ammonites of the Mesozoic. The zonal sequence of many of the genera of southern Europe, and even of some of the species, is repeated in western Cuba, throughout Mexico, and in South America from Venezuela and Colombia throughout the Andean geosyncline. This wide repetition of faunal sequences, beginning in the Triassic and most

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marked during the Jurassic and early Cretaceous, is proof positive that there was a shoreline across the tropical Atlantic, along which these benthonic types dispersed. While the spreading was mainly from east to west in harmony with the equatorial current, yet, as Burckhardt (1930) has pointed out, there are American genera that have spread east into the Mediterranean countries. While the ammonites of the Antil- lean-South American formations show this communication with Europe best, it is also seen, though less strikingly, among the molluscs, corals, and other groups. In his synthetic sti*dy of the Mesozoic formations of Mexico, Burck- hardt (1930) sets forth in many places the intimate ammonite connec­ tions with the Tethyian region of southern Europe and northern Africa. These are noticeable in the late Triassic and very marked in the prolific faunas of the long sequence from the Middle Jurassic into the late Creta­ ceous. They are best seen in the identical and closely allied genera, and more rarely among the species. In the late Jurassic of both sides of the ocean, the genus Nebrodites has its chief development in the Kimmeridgian. Idoceras has some repre­ sentatives in the Middle Jurassic (Oxfordian), but its chief development took place in the Kimmeridgian. Streblites shows in both regions a great expansion in the Kimmeridgian; in Mexico it is unknown in older beds, but in Europe one finds precursors of it from the Middle Jurassic (Cal- lovian) on. Spiticeras begins in Prance in the latest Jurassic (Titho- nian), with the maximum development at the base of the Lower Creta­ ceous (Berriasian). In Mexico the stratigraphic sequence is analogous; in the Upper Jurassic (Portlandian) one finds numerous Proniceras (a closely allied genus), and in the Lower Cretaceous (Berriasian) a great richness of Spiticeras, whereas in the Yalanginian proper there is not the least trace of the genus. In Europe, Astieria is very rare in the Berriasian, has its greatest development in the succeeding Yalanginian and Hauterivian, and vanishes with the latter epoch. In Mexico this genus has a great development in the Yalanginian and is not known in the Berriasian. Streblites and Aspidoceras have spread from Europe to Mexico, and in the reverse direction came Idoceras and Proniceras. In the Lower Cretaceous (Valanginian) the intercommunication is seen in Astieria, Neocomites, Acanthodiscus, Thurmannites, and Kilia- nella. In the younger Barremian there are decided affinities in Desmo- ceras, Puzosia, Pulchellia, Costidiscus, Holcodiscus, and Lytoceras. In late Lower Cretaceous (upper Aptian) time there are in common Puzosia„ Dufrenoyia, Douvilleiceras, Parahoplites, and TJhligella.

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In the Middle Cretaceous, the intermigration is seen in Lytoceras, Ptychoceras, Turrilites, and Hamites. In the more neritic faunas of northeastern Mexico, the following forms show relations with North Africa: Acanthoceras brazoense (near A. aumalense), 8 caphites subevolu- tus (near 8. evolutus), Engonoceras bravoense (near E. thomasi), Euhy- strichoceras remolinense (near E. nicaisei). The intermigration continues into the Upper Cretaceous (Turonian), as seen in Mammites (Metoicoceras), Pseudaspidoceras, Yascoceras, Tylo- stoma, Fagesia, Neoptychites, and Hoplitoides; in the Turonian and the younger Emscherian in Bwrroisiceras, Peroniceras, Scaphites. There are also many genera of molluscs common to the two sides of the Atlan­ tic, as Exogyra, Cerithmm, Actaeonella, Nerinea, Voluta, etcetera. After studying the Triassic cephalopods for more than twenty years, Diener (1915) issued his famous work on the marine provinces of this period throughout the world, which has a paleogeographic map of late Triassic time. He concludes that ammonites in their dispersion follow shorelines, and he does not believe that many forms of these shells were ever floated far by current after death. He thinks that a species may, very rarely, have, crossed an ocean while alive. In the Triassic only 15 genera have a world-wide distribution, and but two species occur very widely dispersed (Ceratites nodosus and Tropites subbullatus). Diener does not doubt that Africa outside of the Atlas lands, and pos­ sibly a portion of the Congo basin, was united throughout all of Triassic time with the whole of eastern and southern South America. The wide similarity of the Norian coral faunas of the Mediterranean and the Pacific geosyncline from California north to Cooks Inlet, Alaska, at 60 degrees north, shows that the faunas must have spread from the east to the west by way of the north shore of Gondwana. In 1911, Uhlig, another student of ammonites, published a world paleogeographic map combining the Jurassic and the older Cretaceous, and following more or less the outlines as determined by Neumayr. This map is here reproduced as figure 2. Uhlig accepts the continent of Gondwana and the peninsula Lemuria. The mediterranean Tethys is continued across the Atlantic area into the Antillean-Mexican region. From the latter area extends the Andine realm divided into two provinces, the southern one extending from Patagonia north to California, and a northern one thence into the Arctic Ocean. Another part of the south­ ern Andine realm is represented in South Africa by the Uitenhage forma­ tion. On the east of Gondwana is the Madagascar-East African province which extends into the Himalayas of eastern Tethys, the latter continuing

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2.— Uhlig’s paleo geo graphic Map of late-Jurassic-early Cretaceous Time, showing E xten t of Continents and various faunal Realms. faunal various and Continents of t xten E showing Time, Cretaceous late-Jurassic-early of Map geo paleo graphic Uhlig’s Printed in 1911. Based in the main on ammonites. From Kayser’s Kayser’s From ammonites. on main the in Based 1911. in Printed “Abriss,” hr eiin 12, ae 393. page 1922, edition, third

898 CHARLES SCHTJCHERT— GONDWANA LAND BRIDGES

southeast into Australasia. To the north of Africa is the vast Mediter- ranean-Caucasus province of western Tethys. In the extreme south and the southeast of the African continent there occurs, according to DuToit (1926), a long series of detrital deposits beginning with the Lower Cretaceous (Uitenhage series) and closing with the Upper Cretaceous (Danian). The Uitenhage series (Sundays Eiver beds) is rich in bivalves and ammonites (Hamites, Hoplites, and sev­ eral forms of Holcostephanus). The Trigonias (9 species) are closely related to those of Cutch, India, and likewise of German East Africa (Tanganyika) and Madagascar, showing connections with eastern Tethys and not with the western part of this mediterranean. Some of this Uitenhage fauna (including Holcostephanus in 3 species) recurs in Bo­ livia, Chile, and Argentina, proving a migration route along the southern side of Gondwana. On the other hand, the flora of the Uitenhage is like that of the uppermost Gondwana series of India, indicating continuous land between those places. • Gregory (1922) presents a paleogeographic map of the Atlantic Ocean and bounding lands, showing a wide Gondwana bridge during Middle Cretaceous (Albian) time.

OTHER MESOZOIC EVIDENCE Echini.—The Cretaceous echini of North America as a whole, accord­ ing to Gregory (1892), present “a very familiar facies to a European echinologist.” Most of the genera are common to the two sides of the Atlantic, but it is only in the Lower Cretaceous of the Gulf of Mexico and Antillean regions that three species are common European forms; these are Diplopodia malbosi, Salenia prestensis, and Pseudocidaris saus- surei. During Upper Cretaceous time the echinoids on either side of the Atlantic began to differentiate and to have their own evolution. This went on until Miocene time, when the echini of the two sides in the same latitudes again took on a common expression, shown in Cidaris melitensis, Schizaster parhinsoni, and 8. scellae. Gregory therefore holds that there must have been in Miocene time shallow-water areas close enough for the echinoid larvae to spread from one island or bank to another. LandI molluscs.—Pilsbry (1911), in his discussion of the living land molluscs of Patagonia, finds much evidence in their ancestral evolution for the existence of Gondwana Land all through the Mesozoic era to near the end of Cretaceous time, when the Atlantic Ocean broke through. These air-breathing and fresh-water bivalves originated back in the late Paleo­

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zoic and evolved and spread throughout the great northern continent of Holarctica and tropical Gondwana as well. On page 632 Pilsbry presents a map showing the migration routes during Mesozoic and Cenozoic time. Marine mammals.—These mammals also furnish evidence of a former Gondwana. Andrews follows this theory and believes that there was an island archipelago here in the earlier Cenozoic. Such a connection be­ tween Africa and South America, he says, “would account for a number of curious facts of distribution” (page 306). He mentions several of these, among them the occurrence of the marine mammal Zeuglodon and the snake Pterosphenus in Alabama and Egypt, showing marine shallow- water connections in late Middle Eocene time. In addition, sirenians occur in both Africa and South America (Egypt has Eotherium in the Middle Eocene). Prom a common source, placed by Abel in the Mediterranean, the dugongs migrated east in the Eocene, and the lamantins west. The former now occur in the Indo-Pacific, the latter in the tropical mes- atlantic region (Florida, Mexico, Antilles, Amazon and Orinoco, Dutch Guiana, western Africa, Senegal, Niger region). The lamantins cer­ tainly could not have traversed the Atlantic Ocean directly, and, on the other hand, their migration was prior to the Miocene (Petit, Compte Rendu, Société de Biogéographie, number 6, 1924, pages 37-38). Land mammals of South America.-—-A detailed statement regarding the known Cenozoic land mammals of South America made by G. G. Simpson to the writer in a letter of April 11, 1932, points out that there is no positive evidence of migrations from Africa during this era. There is, however, good evidence for dispersal from North into South America during the late Cretaceous or earliest Eocene, and much strik­ ing intermigration again took place between these continents during the Pliocene and Pleistocene. Triassic land evidence in South America.-—At the top is the Upper Triassic (Sao Bento) series of igneous and continental formations (2,275- 3,575 feet). Below is the Botucatu sandstone (0-325 feet), likewise of continental origin and having Erythrosuchus, a reptilian genus also found in South Africa. Then follow the Rio do Rasto red shales (325- 1,300 feet), again of continental origin, from which Yon Huene (1929) has collected and described the following rhynchocephalians : Scaphonyx australis, S. fischeri, Cephalonia lotziana, Cephalastron gondwanicum, C. brasiliense, Cephalastronius angustipinnatus, and Scaphonychimus eury- ckorus. These are all of the family Rhynchosauridae, which has 12 spe­

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cies, found in Germany (1), England and Scotland (3), India (1), and Brazil (7). The South American forms Yon Huene refers to the Mid­ dle Triassic and others to the Upper Triassic. Other rhynchocephalians are the Sphenodontidae (2 in Europe) and Gnathodontidae (4 in Middle Triassic of South Africa). The older Permian rhynchocephalians (9) are of the late Permian of South Africa. A cynodont reptile, closely related to those of South Africa, occurs in the same Brazilian formation (Gomphodontosuchus brasiliensis). As Yon Huene is guided by the paleogeographic views of Benson, he concludes that in the early Mesozoic there was at times continuous land from southeast Asia by way of Australasia to Antarctica, and that from there these reptiles spread into South America. To the present writer this Pacific bridge is an impossible one, and he holds it as more probable that the Permian rhynchocephalians, originating in South Africa, spread north into India and Europe, and west via the Gondwana bridge into Brazil. PERMIAN EVIDENCE Folded Pennsylvanian is of wide distribution in the high Andes, and undeformed strata of the same age cover wide areas in the Amazon Val­ ley (the well-known faunas occur along the tributaries Tapajos and Trombetas). In both areas the marine biotas are closely related to those of the western United States. The Permian is of very wide distribution in eastern Brazil and is variable in thickness up to 3,950 feet. It is at the base that the tillites occur, and higher are impure coal beds; the whole appears to be of paralic origin, continental and estuarine, but in northwestern Argentina there are normal marine deposits. In the upper part of the Permian (lower Estrada Nova shales) occurs the later Glossopteris flora. Nest below is the Iraty black shale, with the aquatic reptiles Mesosaurus, Noteoscmrus, and Stereoslernum, which probably lived in both brackish and marine waters. In any event, these black shales have coprolites that include remains of Hexactinellid sponges, organisms that live only in marine waters. The first two rep­ tilian genera recur in South Africa in similar black shales (here with genuine marine invertebrates), showing that these animals had means for dispersal along the southern shore of Gondwana. In the lower part of the Brazilian Permian are the Rio Bonito coal measures with the typi­ cal Glossopteris flora and a few marine inarticulate brachiopods. At the base are the tillites resting unconformably on Lower Devonian forma­ tions or on ancient granites.

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Concerning the wide distribution of the Glossopteris flora, David White remarks:

“The occurrence of this flora in great uniformity, including an extraordi­ narily high degree of specific identity, and in relative purity, contemporaneously in India, Australia, South Africa, and southern South America, leaves no re­ course but to conclude that the land surfaces over which it extended [Gond- wana] were in such continuity, or intimate geographical relation, as freely to permit the migration of the flora, practically in toto, between all these quar­ ters of the globe” (1907, pages 624, 625).

All paleobotanists are agreed that the Glossopteris flora could have been distributed only over land surfaces, and not by means of accidental dispersion, but they are not agreed as to the land bridges. This flora is found in its entirety in India, Africa, Australia, and South America, and parts of it in Kashmir, Afghanistan, Persia, Tonquin, northern Rus­ sia, Siberia, and the Falk] an ds, with indications of it in Antarctica within 300 miles of the South Pole. Some paleontologists hold that the Glossopteris flora migrated across a land bridge from Brazil to Africa and thence to India, and others postulate a bridge from South America to Antarctica and another one-to Australia from there. It is not even known where this flora originated (some paleobotanists think in Antarctica or Australia), but if it arose in South America or in India (the most natural place for the known distribution), it could have spread south into Africa and west into South America and southeast into Australia.

DEVONIAN MARINE EVIDENCE The Helderbergian faunas of the Lower Devonian of eastern North America have more than 450 species. From New York south into Okla­ homa they occur in the calcareous facies and their faunal connection is with Bohemia (Koniepruss). From New York northeast to Newfound­ land, similar faunas are of a more muddy facies, with north European affinities that indicate migrations along the south shore of Eria. The northern facies is of the Saint Lawrence geosyncline, and of Germany and Belgium north of the old Podolian land of central Europe. The early Lower Devonian faunas give rise to the late Lower Devonian (Oriskanian) of eastern North America, and the Coblenzian of Germany; and there is likewise more or less intermigration of these later biotas along the North Atlantic shores. For southern European connections with North America at this time the evidence is not conclusive. How­ ever, the interesting thing about these late Lower Devonian faunas of North Atlantic origin is that they are also found in the Amazon Yalley

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where they are very much like those of the United States, and in the Andine geosyncline of Bolivia and Peru (for more detail, see Schuchert 1921, pages 343-347). Everywhere they are characterized by the brachio- pods Leptocoelia flabellites and early terebratulids. These are all of the North Atlantic realm, of the ocean Poseidon to the north of Gondwana Land. On the other hand, the late Lower Devonian of Bolivia, Argentina, Falkland Islands, and Cape Colony, according to Clarke— “bears a special and distinctive impress which is characterized as austral in contrast to the boreal aspect of homotaxial faunas north of the equator. These distinctions consist in specific resemblances without identities; in parallel developments” ; and in strange generic intrusions (1918, pages 7-8).

The late Lower Devonian of Brazil in Clarke’s boreal realm has about 170 species, while that of the austral realm in Argentina (San Juan area) has 10 forms, in Bolivia 80, and in the Falklands 30. The contempora­ neous fauna of South Africa (Bokkeveld) has an assemblage of 186 spe­ cies; of these 40 are also found in South America, and 71 have close allies there. Prom Clarke’s studies we may conclude that western Gondwana had a distinct Lower Devonian marine fauna along its southern side, and that there was an equally well differentiated one along the northern and western shore of this equatorial continent. The reason why the Lower Devonian faunas of northern Europe and North and South America have so much that binds them closely together is because the ocean of these areas gave them communication with one another. This is strikingly shown in the great pelecypod development of the Coblenzian of the Bhine region, where there are genera on genera that appear in America in the Middle Devonian (Onondaga and Hamil­ ton). SILURIAN MARINE EVIDENCE The Middle Silurian of Arkansas and Oklahoma has a rich fauna in the Saint Clair limestone. H. S. Williams was the first to note its Euro­ pean affinities, and these were seen later and more clearly by Van Ingen. Ulrich and Mesler (1925) have assembled an extensive collection of nearly 200 species which they say represents one of the most interesting faunas in America. The fauna as a whole is “much more closely allied to the Silurian fauna of Bohemia than to the usual American, British, and Gotlandian faunas of the same period,” the affinity being especially close among the brachiopods (50 + species), molluscs (50), and trilo- bites (45). It is an assemblage unlike any other, from the American

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Silurian in that it is so distinctly European in its origin. TJlrich and Mesler do not say how this fauna got from Bohemia into the southern United States, but there is only one available route, via the north shore of Gondwana Land. The Bohemian sea lay south of the Podolian axis, which makes the northern shore of Tethys, while the assemblages to the north of the same axial land spread via Eria and down the Saint Law­ rence geosyncline into the northern United States. The Silurian of the Amazon Valley is widely distributed along the tributaries Trombetas, Curua, and Maecuru in Brazil. The fauna, de­ scribed by Clarke (1899) from the Trombetas Valley, is a small one of about 20 species, and yet it has the familiar North American burrow known as Arthrophycus harlani, which is also common in the Ordovician of Spain. Clarke noted other relationships with the Silurian of the United States (chiefly the Medina), shown in at least 5 closely allied forms. It is highly probable, therefore, that this Silurian fauna also migrated along the north shore of Gondwana and from there spread into the United States and the Appalachian trough.

P a r t III.—E v id e n c e f r o m l a r v a l L i f e

DISTRIBUTION OF LARVAE IN GENERAL As it is impossible, during adult life, for the great bulk of any shal- low-water marine invertebrate fauna to spread of its own volition across the deeps of an ocean, like the Atlantic, from south Europe and north Africa to Brazil and the West Indies, we must now inquire if any of it could be transported alive by an oceanic stream such as the North Equa­ torial Current, which goes from east to west. This may be accomplished in some cases by such swimming animals as fishes or crabs, and possibly in rare cases by ammonites, because they can feed while drifting across. However, very little of the shallow-water adult life can depart far from the shelf seas because of the lowered temperatures of the deeps, the great pressure, and the scarcity of available food. On the other hand, all of the invertebrates reproduce by eggs and in most of the species these give rise to freely floating microscopic larvae that rise to the surface, where they live and develop and are drifted about by the currents. In this way, larvae are caught by the Equatorial Current off Africa and carried out into the high seas, where the cooler waters retard their development. Can any of them be drifted alive across the Atlantic for 4,000 miles before again encountering shallow sea-bottoms on which to grow up to maturity? The following pages give the answer, which is that very few indeed can do so in one passage.

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DISPERSAL OF MARINE INVERTEBRATES Each marine species, says Professor MacBride— “has a particular type of bottom which is suited to it, and which may be termed its home. On ground of this kind it swarms ; but around the areas of this type of bottom numerous stragglers of the species are to be found. It seems clear that, from the home population, crowds of colonists are forever being sent forth which, in most cases, fail to maintain themselves, but which may in rare cases successfully establish themselves, and in this way a new race or species may be produced” (1914, page 630).

Different species of plants and animals arise in different habitats of either the fresh waters or the dry lands or of the seas and oceans. Once established at one point, each kind will spread as far as its environment goes, or as far as it can adapt itself to the conditions present. It is only barriers of some sort that will stop this dispersion, and in the seas and oceans these barriers are land, temperature, food, depth, salinity, etcetera. The species of a flora or fauna, whether of the living or the fossil world, have a variously wide distribution. At least 5 per cent of living marine invertebrates have coastline ranges of from 5,000 to upward of 10,000 and even 15,000 miles, and since about 60 per cent of bottom-dwelling forms have ranges of between 2,000 and 3,000 miles, this variably wide distribution shows with certainty that the provinces and realms of the geologic past can also be ascertained (Schuchert, 1921). In the seas and oceans, all the life that lives on or crawls over the bottom, or burrows in it, or is attached to it, or is dependent on it for food is embraced under the term benthos. The life that swims or floats about at the surface is called the plankton, while the actively swimming animals of the intermediate realm are of the nekton. The term benthos, however, usually refers to the bottom life of the shallow waters border­ ing the continents and islands, or around the oceanic islands and sub­ merged banks of the oceans. In the temperate and colder water the benthos is limited roughly to 600 feet of depth, though much of it goes down the continental slopes to 1,000 feet or more, and in the tropics to 1,500 feet, because here sunlight penetrates to greater depths and ac­ cordingly there is also some plant life, the basic food for all animals. The variety of microlife in the plankton is extraordinary, and it in­ cludes animals in all stages of growth, most of which live permanently or seasonally in this habitat, but to them are added each year during the warmer months by the oceanic currents a vast abundance of eggs and developing young (larvae and other stages) of the bottom-living ani­

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mals of the shallow and marginal seas. This addition is known as the transitory plankton. Temperature is an important factor in the dispersal of animals. A quick change of a few degrees to below the normal for the spawning season, produced by a heavy downpour of cold rain for a day or two, will, for example, destroy every last floating larva of oysters. Hence in the long run larval life will spread farther in equatorial and tem­ perate seas than in boreal and austral ones. During most of geologic time the climate was more equable than now, and hence many more kinds of life could spread much farther north and south. Indeed, at certain geologic times the evidence of reef corals and other warm-water animals is found in far northern waters, and limestone—indicative of warm seas—occurs around the North Pole. Moreover, latitudinal changes were not so effective in re­ stricting life to climatic belts as is< the case today. What we now see in the distribution of the warm-water seas was the general condition over most of the earth during the greater part of geologic time. To­ day the shallow-water marine life of the Atlantic is very much alike from Pernambuco, Brazil, to Florida, and some of it goes even to Ber­ muda and Cape Hatteras, but under a more equable climate it would be about the same to Greenland and Norway. The spreading of the eggs and larvae of the benthos across the oceanic depths is mainly a question of how many days this minute life can live in the plankton. In the earlier stages these young have no mouth and are dependent on the yolk provided by the mother—an amount that will soon be consumed—and until a mouth and a functioning gut are developed, they can not live more than a week. But feeding larvae can live longer, and some kinds, like the crustacea, may, under the most favorable conditions, live for two or three months, and possibly even longer. In the meantime they may have drifted 500 to 1,000 miles at the rate of 8 to 16 miles per day, but the normal chances of distribution will be far less. Then comes the time when they must forsake the plankton and seek the benthos, their further life and development de­ pending on their finding depths of less than 1,000 feet, the proper tem­ perature, and, above all, the right kind of bottom; in other words, they must find, through accident, the environment under which their ances­ tors grew up. Under these circumstances it is readily seen that nearly all of the life of the benthos lost on the high seas is doomed long before it can drift across an ocean or even a body of water over 1,000 miles across. Should the basin, however, have ridges and banks with shoal

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water, or if there are volcanic islands properly spaced, it is plain that much of the life of one shore can get to the other through larval dis­ persion, provided there are suitable currents to take it across; but in that case the life from the other side can not as a rule cross over against the currents. The oceanic island of Bermuda is a brilliant example of how such places can be peopled by a waif fauna, showing that the larvae of the bot­ tom marine life of the far-away Bahamas and the Florida Keys are swept out into the Atlantic by the Gulf Stream, and after being transported 650 miles have peopled the shallow waters over the top of this extinct volcano. None of this life, however, reaches the Azores.

DURATION OF LARVAL LIFE General.■—The writer asked Professor E. G. Conklin about the dura­ tion and distribution of marine larval life in general, and he kindly replied as follows, under date of April 2,1932: “Practically all sedentary invertebrates have free-swimming larvae, and these, as we know, may be carried far by ocean currents, for example, in the case of numerous invertebrate and vertebrate tropical forms which are found during the summer off the coast of Martha’s Vineyard and Nantucket. In the case of Anthozoa I know that the larvae may be carried to great dis­ tances—indeed all the way from Tortugas to Nantucket, a distance of not less than 1,500 miles. Similarly the larvae of many gastropods live for a considerable period as such. The free-swimming larval Crepidula plana exist in this state for at least a month, and are carried to great distances by currents and winds. . . . “But even granting such a long larval period to many forms, it does seem impossible to explain the distribution across a great ocean basin 4,000 miles or more in width. I am inclined to think, however, that in such a case as this, the explanation could be found in the former existence of islands in such ocean basins rather than of a continuous land bridge. If there were formerly such islands in the north Atlantic lying between Bermuda and the Azores, it would be possible to explain the distribution of many of the marine forms which have free-swimming larvae, across the present Atlantic. However, this would not explain the distribution of fresh-water and land forms, and these are after all the crucial tests of former land connections.”

Truly so, and it is on the distribution of this land life that most of the land bridges have been constructed. The invertebrate paleontologist sees nothing of this larval life, and there are many classes of soft-bodied animals of which he rarely finds fossil evidence, and so in our further consideration of the distributioii of marine life it is only necessary to consider the corals, echinoderms, brachiopods, molluscs, and crustaceans.

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Corals.—The eggs of corals develop into planulae. According to Gardiner (1904), the planulae in the South Pacific around Rotuma and Funafuti settled to the bottom in from 5 to 8 days. His observations led him to conclude that in no case could they be directly carried from Ceylon to the Maldives in the Indian Ocean, though he thinks it conceivable that these larvae might be swept from reef to reef via the various Lacca­ dive banks, and so reach that group. Vaughan (1916) studied the larval growth of four species of corals at Dry Tortugas in the Gulf of Mexico. These larvae were in the free- swimming stage from 2 to 23 days. In a current going continuously in one direction at the rate of 3 knots per hour the planulae might be car­ ried 1,656 knots = 1,900 statute miles. This carriage, however, is not a natural one and it is doubtful if they would go farther than 1,000 miles. He comments on the fact that every species of shoal-water coral in the Bermudas is found in Florida and the West Indies, and thinks that the clue to the wide distribution of living coral species is given by the pos­ sibly long duration of the free-swimming larval stage. Elsewhere Vaughan says that some of the species off the east coast of Africa also occur in the Hawaiian Islands; in this case there must have been inter­ mediate stations of dispersal. Echinoderms. General.—This group embraces the starfishes, ophiu- rans, sea-urchins, holothurians, and crinoids, all of which are exclusively marine in habitat and most of which live in the benthos. But few kinds of adult echinoderms swim about. In many of these animals the larvae rise to the surface, are drifted about by the currents, and so get into the plankton. Here they may live anywhere from 2 to 10 weeks before they seek the benthos. Echinoids (sea-urchins).—In regard to artificial rearing of sea-urchins, Professor MacBride writes me as follows on April 13, 1932, with the proviso that such experiments are open to the objection that in real life the larvae might develop much more quickly than in confinement: “For the last sixteen years I have been rearing the larvae of Echinus miliaris through their metamorphosis. In general 1 find that the length of time re­ quired is 6 weeks, but I have known it to be as short as 3 weeks and as long as 10 weeks—and the length of time depended on the rapidity of growth of the larva and this in turn on the abundance of the diatom food present. 1 have twice reared the larvae of our common starfish Asterias rubem and they swam for two months. I have reared the larvae of the heart urchin McUim- cardium and they required about 3 weeks to metamorphose. Also on one occasion I reared the larvae of the brittle star Ophiothrix fragilis and they also required 3 weeks to go through.”

LIX —B u l l . G e o l . S o c . Am., V o l . 43,1932

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Many of the living echinoids are viviparous and therefore have no free-swimming stage. The complete life cycle is known only in a few species (see MacBride, 1914, page 504). Blochmann (1908) reports that Mortensen caught single specimens of echinoid larvae in the middle of the Atlantic Ocean, and that all were feeding larvae. Agassiz has also taken the same species of deep-sea larvae of echinoids on both sides of the Atlantic due, he thinks, to current dispersion, and he has captured young echini in the plankton. Accord­ ingly, it appears possible for deep-sea echinoid larvae to spread across the entire Atlantic, but the littoral species can rarely or not at all disperse so widely unless there be intermediate shallow-water banks or islands. At Plymouth, England, Allen and Nelson (1910) reared in tanks three species of Echinus. E. esculeptus and E. acutus metamorphosed in from 6 to 9 weeks after fertilization. Mortensen (1910) lists 82 species of echinoids in the Atlantic of North America and the West Indies; one is circumpolar in distribution, and another is cosmopolitan. Under date of April 15, 1932, R. T. Jackson writes that, as shown by the memoirs of A. Agassiz (1883), Mortensen (1927), and H. L. Clark (1925), about 23 species of living echini occur in the West Indies that are also found elsewhere on the eastern side of the Atlantic or in the Mediterranean; and of these 7 are shallow-water forms, the rest being of deep seas. Hence it is evident that even some littoral echini have a very wide distribution, and that such wide dispersal is especially true for the deeper-water forms. Starfishes.—The following is taken from MacBride (1914). The embryonic and larval stages are well known in Asterina gibbosa (em­ bryonic life 4 to 5 days), Cribrella oculata, and Solaster endeca. The first has a larval mouth and the other two have none. In Asterias, Astro- pecten, Luidia, etcetera, the larva feeds and lives a free-swimming and self- supporting life for a long period, often extending over 2 months, until at last it metamorphoses into the adult form. Gemmill reared the larvae of Asterias rubens for over 2 months in his Glasgow laboratory, until they had completed their metamorphoses; and Delage did the same with the larvae of A. glacialis at Roscoff. Ophiurans (brittle-stars).—Development of these animals is known in Ophiothrix fragilis, Ophiura brevispina, and Amphiura squamata, only the first of which has a long larval development comparable with that of Asterias. Ophiura brevispina passes through the larval stages in 10 days and Ophiothrix fragilis in 3 weeks.

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According to H. L. Clark (1911, paper not listed), the distribution of ophiurans in the oceanic fauna shows that species occurring on both sides -of the North Pacific have a great bathymetrical range. Of such there are 14 forms and they range from shallow water down to 1,800 feet and three go even to 9,000 feet. This wide spread horizontally and ver­ tically is probably due to a long larval life and favorable currents for dispersal. Bryozoa.—The free-swimming stage of bryozoans is thought to be limited to a few’hours and at most a few days, and curiously these animals are often of great geographic range. After their short larval life, they are wholly sedentary in habit. Brachiopods.—Brachiopods are not rare as living species in the pres­ ent seas and oceans, Thomson (1927) recording 183 species in 59 genera. Of these 40 are of the group Inarticulata in 7 genera, and the rest belong to the Articulata. Of the 59 genera, 24 are restricted to less than 150 fathoms, but of all the forms confined to this depth there are 143 in 47 genera, and the majority of these 143 species live in waters of less than 100 fathoms. , Between 200 and 500 fathoms live 34 genera (6 restricted to this depth) with 64 species, and many of these are also of shallower or deeper waters. Between 500 and 1,000 fathoms there are 15 genera with 21 species, of which 12 also live in depths less than 100 fathoms. Of genera living at greater depths than 1,100 fathoms there are 12, with 18 spe­ cies ; and at over 2,000 fathoms there are 6 genera with 7 species. The above figures show that the bulk of species and genera of brachio­ pods live in shallow water, and that they become less numerous in greater depths, but occur at all levels down to 2,737 fathoms (Pelagodiscus atlanticus). In some brachiopods, according to Blochmann, the early development takes place in brood pouches. Lingula and Discina have occasionally been taken as pelagic larvae having a mouth and functional gut and there­ fore self-sustaining. Yatsu (1902, paper not listed) says that the larvae of Lingula anatina of Japan may live in the free-swimming stage in aquaria for not longer than 2 months, during which time they feed on diatoms and other unicellular algae. According to Thomson, the larvae of inarticulate brachiopods can be widely distributed by currents, and yet it is not among them that one finds the widest geographic distribution, with the one exception of Pelagodiscus, a deep-sea species. Oif Norway, where a few articulate species are common, Blochmann has never captured a pelagic larva, though his aquaria had them by the

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hundreds. In deep undisturbed water the larvae do not swim far from their mothers, but in shallow waters they are dispersed by the currents. Larvae of the Articulata appear to be excessively rare in the plankton, and this suggests to Thomson that the free-swimming stages are of short duration, and that normally they remain near the bottom. None of the larvae of articulates have a mouth or stomach, and yet it is among them that the adults have the widest distribution geographically and bathy- metrically. In aquaria, Terebratulina septentrionalis and T. caput-ser- pentis are free up to 14 days; none are known to rise into the plankton. Apparently, therefore, no shallow-water articulate brachiopod can cross an ocean, though they are known to have spread very far along coast­ lines. In the Equatorial Current, Blochmann figures that brachiopod larvae may under favorable conditions spread from 30 to 60 miles per day, but even at this speed the African larvae could not reach Ascension alive, since such a passage would require from 31 to 62 days. Mollusca.—Probably the most characteristic animals of the shelf seas are the molluscs (bivalves, gastropods, and cephalopods). Here they live attached to, or crawl or swim over, or burrow in, the sea-bottom, but almost all reproduce by means of eggs and many of these rise to the surface to appear also in the transitory plankton. Some gastropods are viviparous. The number of eggs is usually very great in the gastropods, and in the bivalves there may be millions. Most molluscs produce young that do not become pelagic and yet many kinds do have larvae that rise and float away into the plankton. It is by the production of these free- swimming larvae that the molluscs are able to distribute themselves widely and quickly. Gastropods frequently do not have free-swimming larvae, since they develop their eggs in capsules that are usually fastened to the sea-bottom; there are, however, many forms that have pelagic larvae. The larvae of Ostrea virginica float up to about 6 or 7 days and a day or two earlier develop a mouth and begin to feed before sinking to the bottom to cement themselves to some hard object above the mud bottom (Kellogg, 1910). Stafford (1913) has reared the Canadian oyster from the earliest egg condition through larval life into the spat form that settles and fastens to the sea-bottom. Many of the eggs remain on the ground and others rise in the water and so are distributed about. In 5 to 9 hours the eggs grow into larvae (Brooks got the same results in 2 to 11 hours). The larvae become fixed in from 1 to 5 days. Huxley kept larvae for a week, Rice for 14 days, Lacaze-Duthiers for 30 and even

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43 days without apparent change. Finally, Stafford concludes, “the length of the period of an oyster’s free-swimming life may be considered to be between 3 weeks and a month” (page 84). My a armaria and Venus mercenaria have a larval life of 3 to 6 days, and the same appears to be true of some scallops (Pecten). The gastropod Patella in the post-trochophore stage was kept alive for a week, hence the whole larval life is longer.' In the English Fisheries Department at Plymouth, larvae of the cowrie have been kept alive for a month or more, and when about 1.25 millimeters across are ready to go into the benthonic mode of life (Lebour, 1932, Natural History, vol­ ume 32, page 193). Conklin (1897) has studied the complete life history of the fixed gas­ tropod Crepidula, which lays its eggs in capsules. In 0. plana and 0. fornicata development takes— “about 4 weeks from the time the ova are laid until the fully formed veligers escape from the egg capsules, and in C. convexa and G. adunca the period pre­ ceding the escape of the young is probably much longer. How long the veli­ gers of the two former species lead a free-swimming life I do not know. . . . In C. fornicata the veligers do not swim about for more than 3 weeks, prob­ ably about 2. . . . The whole course of development, therefore, from the time the eggs are laid to the close of the larval life and the assumption of adult characters and habits, is from 6 to 8 weeks” (1897, pages 17, 18).

Odhner in a letter to the writer of May 11, 1932, says his studies of the larvae of Natica show that they live as such for a month or more near the sea-bottom and swim but little, accordingly they can not be dis­ persed far. Probably the same is true for Capulus. Lebour has kept larvae of Nasscmm reticulatus for 2 months, and of Trivia europaea for 5 weeks, but even so, this larval duration is far too short for gastropods to cross the Atlantic alive. Odhner writes that he is convinced of the former existence of transverse oceanic land bridges. Crustacea.—In the plankton of the oceans no type of animals is more common than the adult crustaceans, which occur in “countless forms and individuals.” In the oceans they play the role that the insects do on the lands. These common crustaceans are the Copepoda (most abun­ dant), Ostracoda, and Cladocera. Among the larger crustaceans, the Schizopoda, the Amphipoda, and the Decapoda are also very important, but of much less abundance and specific variation (Hjort, 1912). The great bulk of the crustaceans live in the littoral region. Tem­ perature is the main cause of their geographic limitation, and not the other ordinary barriers of the sea. The circumtropical zone has many

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species in common with the Atlantic and Pacific coasts and the Indo- Pacific. These are of the Grapsidae and Albunea, also of the tropical land crabs. This wide distribution is thought to have been helped greatly by the Panama portal. Gardiner (1904) has kept larval crustacea alive and unchanged for 12 days, but thinks that in the seas and oceans they might live and evolve for at least 30 days. He is sure that— “wherever any bank may appear in the Indo-Pacific or Atlantic Oceans it should be speedily provided with a fauna of such Crustacea as possess free- swimming larvae of the zooean and more developed types” (1904, page 407).

As nearly all crustaceans lead a free and unattached life, crawling, swimming, and drifting about, their mode of life is conducive to wide dispersion, and when to this is added the more or less long floating larval life, it is plain why the genera and species of the group are so widely spread. The decapods produce great numbers of eggs—a single female crab may carry about from 5,000 to 10,000—and out of them develop the zoea larvae, which— “have a comparatively long life in the sea as pelagic animals before they undergo their first metamorphosis into the Megalopa stage, and the megalopa has also a somewhat long life in this stage. By-and-by the megalopa, by repeated moults, passes into the adult stage and the Decapods then settle down to the sea bottom for the remainder of their natural lives” (Johnstone 1908, page 70).

About Bermuda, Yerrill (1908) notes 78 species of decapods and of these 72 (93 per cent) are also found in the Florida Keys or the West Indies, demonstrating the close faunal relations of the two regions. About 53 of the forms (68 per cent) range from Florida to Pernambuco, Brazil, or further south. About 25 species go to Cape Hatteras and 6 to southern New Jersey. “Several species of crabs and shrimps habitually live among floating sar- gassum, or attached to floating driftwood. . . . But the majority of the species common to Bermuda and the West Indies . . . must have migrated northward in the free-swimming larval stages. The directions of the Gulf Stream and prevailing wind currents are favorable for the transportation of free-swimming animals from the Bahamas, Cuba, etcetera, to the Bermudas” (pages 294-295).

This distribution, Yerrill points out, could not have gone in the op­ posite direction against the flow of the Gulf Stream, and besides, no European or Mediterranean species of decapod occurs in Bermuda.

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“It is evident, therefore, that the Bermuda decapod crustacean fauna is an offshoot or colony from the West Indian fauna, with only a slight admixture of species from other regions. In this respect the Crustacea agree with the Anthozoa, Mollusca, Echinoderms, etcetera” (page 291).

B ibliography

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