The Atlantic Ocean Biologically an Inland Sea.

BY Austin H. Clark, (Uriited States National Muscum.)

It has been suggested by several geologists, notably by Professor Sues s, that the Atlantic Ocean is not a true ocean like the Pacific, but is a basin of secondary and of comparativly recent formation; and Professor W o e k o f f has recently shown, in a most interesting contribution, that meteorologically also the Atlantic should be con- sidered as an inland sea rather than as a true ocean. The question therefore naturally arises, is the Atlantic Ocean biologically an inland sea? Before we can answep this question we must define an inland sea in terms of biology. An inland sea, biologically speaking, is a more or less enclosed body of water which, connected with an ocean, has received all of its fauna from that ocean. Its fauna, therefore, is composed of the same types that occur in the ocean with which it is most intimately connected, with the less plastic and adaptable weeded out an-d the remainder modified in proportion to the difference between the physics and chemistry of the inland sea and that of the parent ocean. All inland seas necessarily 'differ physically to a greater or lesser degree from the omans 'with which they are connected. Their abyssal water cannot form a part of the general abyssal circulation of the oceans, moving slowly anticlockwise about the oceanic basins, and therefore tends to become more or less stagnant and, under certain conditions, very cold. Their surface water, no longer a part of the general superficial oceanic circulation, unless there be an outlet 2 Austin H. Clark. sufficiently large so that a continuous flow is maintained, increases in salinity through an excess of evaporation as in the Mediterranean and in the Red Sea, or decreases in salinity through an excess of rainiall in the tributary drainage area as in the Baltic. Either of these changes is fatal to a certain proportion of the organisms which enter inland seas, so that necessarily their fauna is composed only of the more resistant or adaptable organisms of the parent oceans. On account of the physical alteration of the waters of an inland sea, through which they become less favourable than oceanic waters for the support of marine organisms, inland seas never serve as the cradle for new organic types; their fauna is entirely derived from outside, though the component elements may be forced to undergo a certain amount of modification in order to meet the new conditions imposed. An inland sea of the present epoch may be a derivative from a much greater sea of the past, as in the cass of the Mediterranean. The restriction in size of any large portion of an ocean immediately alters and restricts the circulation of the enclosed water, bringing it more and more under the influence of the local meteorological con- ditions. The effect upon the fauna is therefore exactly the same as though the sea arose through a sinking of the land resulting in an inflow of oceanic water. The biological conditions in an inland sea are not in any 'way concerned with the question of whether the sea originated as a sinking of the land, or whether it arose as a restric- tion of a previously much larger body of water. Both processes lead to a mean which is physically land oceanographically the same, and therefore biologically the same. A number of curious types occur in enclosed seas which &re quite different from any types inhabiting the oceans with which these enclosed seas were once connected. These types are mainly to be interpreted as relics of a once generally distributed fauna which, able to survive the changing conditions, have been preserved from extirmination by the fact that none of the economically more efficient types of later origin, through competition with which they have been extirpated from the oceans, have been able to enter the enclosed basins, for the reason that these basins became cut off from the oceans before the appearance of these later types. The Atlantic Ocean Biologically an Inland Sea. 3 Such types are found in enclosed seas, but almost never in inland seas, for the reason that all types of later origin are excluded from the former; an animal type efficient and vigourous enough to over- come and exterminate competing types in the oceans would, other things being equal, also be efficient and vigourous enough to extir- pate them from all 'the inland seas. The existence of 'such types in an inland sea furnishes conclusive proof that some sort of a barrier is operating to keep out of that inland sea possible competing forms from the adjacent ocean so that, so far as this type is concerned, the inland sea is in reality an en- closed sea. Anticipating for a moment, it may be mentioned that among the crinoids two such types occur in the Caribbean sea, I so c r i nus and- H o 1 o p us. These persist here not for the reason that they originated here, but because the disruption of the connection between the Carib- bean region and the East Indies took place before the evolution of the more efficient and vigorous types now dominant in the Indo- Pacific littoral, through competition with which they have there been extirpated. In the following list are given, in the left hand column, all of the crinoid genera inhabiting the Atlantic basin, with their ascertained ranges; and in the right hand column the corresponding (or equi- valent) Indo-Pacific genera, also with their ranges. Suborder Oligophreata. Family C om as t erid a'e. Subfamily Capi 11 asteri n n e. N eo coma t ell a. C om atel la. Caribbean Sea; southwestern Indo-Pacific region, from Japan Europe; northwestern Africa. to Australia, Ceylon and south- eastern Africa. Nemaster. Capil last er. Bermuda, the Caribbean Sea, Indo-Pacific region, from Japan and southward to Brazil. to Australia, Ceylon and south- eastern Africa. Leptonemaster Corns sia. Comatilia Caribbean Sea. Red Sea southward to south- emtern Africa, thence eastward to the Philippine Ishds. 4 Austin H. Clark.

Subfamily Co mac tiniin ae. Coma c t in is. C o m atul a. Southeastern United States and Australia northward to the southward to Brazil. Philippine Islands and Hong Kong, westward to the Anda- man Islands. Family Colo b ome tridae. An alcid ome tra. Olig ometrides. Caribbean Sea. Southeastern Australia north- ward and westward to the Andaman Islands. Family Tr o pi o m e t r id ae. Tropi o me t r a. T r opi o m e t ra. Ciuibbean Sea, and southward Entire east coast of Africa, and to Brazil. eastward to Australia and Japan.

Family T h alasso met r id ae. C r o t a1om e t r a. Cr o t alomet r a. Caribbean Sea and Bay of South Africa and .the Laccadive Biscay to Ascension Island and Islands to the Malay Archi- South Africa. pelago. T h a1ass o m e t r a. T h a1as s o me t r a. Portugal and southward to As- Arabian Sea and the Crozet cension Island and Tristan da Islands, eastward to the Ker- Cunha. madec, Hawaiian, Galapagos and Aleutian Islands and Japan. StyI ome t pa. C o s mi ome t r a. Carribbean Sea. Southeastexn Africa to the Malay Archipelago , the Hawaiian Is- lands and Japan. Family Charitometridae. Pachylometr a. 4: r ino m e t r a. G1yp t o m e t r a. Caribbean Sea. Arabian Sea and eastward to the Malay Archipelago , the Hawaiian Islands and Japan. The Atlantic Ocean Biologically an Inland Sea. 5 Suborder Maerophreata. Family Anted o n id a e. Subfamily Antedoninae. Mast igo me t r a. Anted o 11. E u an t e do n. West Indies to Brazil; Norway Ceylon to the Malay Archipelago to tho Gulf of Guinea; Mediter- and Tahiti. ran ean. Subfamily Pero m e trin a e. H y p alome t r a. P er omet r a,. Caribbean Sea. Southeastern Africa to the Malay Archipelago and Japan. Subfamily Zen om e tr in a e. Zenometra. Zeno m etra. E um orph o m e tr a. Caribbean Sea. Extreme south of the Indian Ocean to the Hawaiian Islands. L ept om e tra. P s a t h yr o m e tra. Madeira to the Hebrides; Me- Bay of Bengal and the antarctic diterranesn. regions south of the Indian Ocean to the Hawaiian Islands, the Gakpagos Islands, the west coast of North America, the Aleutian Islands and Japan, including the Sea of Japan. A d e lom e tra. h d el o metr a. Cuba. Ki Islands. Subfamily H e lio me t rin a e. Heliometra. C y clo m e tr a. Arctic Ocean, and southward Arabian Sea to Japan. to Nova Scotia and the extreme north of Europe. 'l'richome tra. Trichometra. Caribbean Sea to the Grand Bay of Bengd and the antarctic Banks of Newfoundland; Bay of regiocs south of the Indian Biscay to Ireland. Ocean to the Sunda, Philippine and Hawaiian Islands. 6 Austin €1. Clark.

Hathr o m e t r a. Trich o metra. Arctic Ocean, and southward (see above.) to Chesapeake Bay and Portu- gal; antarctic regions south of the western Indian Ocean. Is om e t ra. V) Off Montevideo, and southward to the Burdwood Bank. Subfamily Th ysanometrin ae. C o ccome tr a. Thysan om e tr a. Caribbean Sea. Admiralty Islands to Japan. Subfamily Bathy m e trin ae. T h aumat o m e t r a. T h aumat om etr a. Southwest of Iceland. Arabian Sea and southnwd to the antarctic, thence throughout the East Indian region to the Galapagos Idands, Central Ame- rica and Japan. Bathy met r a. T3 a thymetr a. Off the Abrolhos Islands, Brazil. South of Tasmania northward to the Galapagos Islands and the Aleutian Islands.

Family Pentam e tro crinidae. T h au m ato crinus. T h aum at o crinus. Southwest of Iceland. Antarctic regions south of the Indian Ocean and Australia; Meangis and Hawaiian Islands, and Japan. P e n t amet r o crinus. P ent ame t r o c r i nu s. Canary Islands to Ireland; Arabian Sea to the East Indies Caribbean Sea. and Japan. Family Atelecrinid ae. Atelecrinus. Atele crin us. Caribbean Sea, and southward Sunda Islands and the Malay to Brazil; western Ireland. Archipelago to the Hawaiian Islnnd s. The Atlantic Ocean Biologically an Inland Sea. 7

Family P en t ac1 ini t id a e. Endoxocrinus. End ox o crinus. Caribbean Sea and southward to Sunda, Philippine, Meangis and Brazil; from south of the Ca- Kermadec Islands. nary Islands north to Rochefort. I socrinus. 0) Family Bour guetier in id a e. Bathycrin us. Bathycrinus, Mid-equatorial Atlantic 10 the Eastern part of the Bay of coast of Maryland. Bengal to the mid-Pacific be- tween Oceania and America. Mo nach o crin us. hI o n a c h o crin u s. Caribbean Sea to the Azores Bay of Bengal, and the vicinity and Morocco, and northwest- of Banda and Celebes. ward to southwest of Iceland. Rhiz o cri n us. (‘4 North Atlantic from Florida and southern Ireland northward. Bythocrinus. By t ho cr i nus. Caribbean Sea and southward Atlantic coasts of northwestern to Ceara,, Brazil; Atlantic coasts Africa and southwestern Europe; of northwestern Africa and Northeastern Africa to the East southeastern Europe. Indies. Democrinus. Demo crin us. Caribbean Sea; coast of Morocco. Sulu Archipelago, Timor and Ceram Laut.

Family H 010 p o did ae. Ho 1 opus. 0) Caribbean Sea. Family Plic a t o cr inid a e. Gephyro crinus. Th alassocrinus. Canaries and Madeira. Celebes. I. Atlantic genera finding their nearest relatives in genera ranging from east Africa to the East Indies. a) Atlantic genera finding their nearest relatives in genera occurring in the Arabian Sea and eastward. 8 Austin H. Clark. Arabian Sea and eastward to the Malay Archipelago, the Hawaiian Islands and Japan. Crinometra. Caribbean Sea. Arabian Sea to Japan. H eliom e t r a. Arctic Ocean and southward to Nova Scotia and the extreme north of Europe. Pentametrocrinus. Bay of Biscay to Ireland; Cnrib- bean Sea. Arabian Sea to the East Indies. R ytho crin us. Caribbean Sea and southward to Ceara, Brazil; Atlantic coasts of northwestern Africa and southwestern Europe. b) Atlantic genera finding their nearest relatires in genera occurring in southeastern Africa and eastward. Southeastern Africa to the Malay Archipelago, the Hawaiian Islands and Japan. Stylom e t ra. Caribbean Sea. Southeastern Africa and Ceylon to Australia and Japan. N e o c o m a t ella. Caribbean Sea; southwestern Eu- rope and northwestern Africa. N e m aster. Bermuda, the Caribbean Sea, and southward to Brazil. Southeastern Africa to the' Malay Archipelago and Japan. Hypalo metra. Caribbean Sea. Southeastern Africa and the Laccadive Islands to the Malay Archipelago. Cr o t a1 o m e t r a. Caribbean Sea and the Bay of Biscay southward to Ascension Island and South Africa. c) Atlantic genera finding their nearest relatives in genera occurring from the Red Sea to southeastern Africa and east ward. Red Sea and southward to southeastern Africa, thence eastward to Australia and Japan. Tropiometra. Caribbean Sea and southward to Brazil. The Atlantic Ocean Biologically an Inland Sea. 9 Red Sea and southward to southeastern Africa, thence eastward to the Philippine Islands. L ep t o n em ast er. Caribbean Sea. Co m atili a. Caribbean Sea. d) Atlantic genera finding their nearest relatives in genera occurring from the Red Sea to the antarctic regions, thence eastward at least to the Galapagos Islands and Japan. Arabian Sea southward to the Crozet Islands, thence eastward to the Kerniadec, Hawaiian, Galapagos and Aleutian Islands and Japan. T h alas s ometr a. Portugal and southward to Ascen- sion and Tristan de Cunha. Arabian Sea and southward to the antarctic, thence eastward to the Galapagos Islands, Central America and Japan. Thau m a t o m e t r a. Southwest of Iceland.

11. Atlantic genera finding their nearest relatives in genera which do not occur west of the Bay of Bengal. a) Atlantic genera finding their nearest relatives in genera which occur west to Ceylon. Ceylon to the Malay Archipelago and Tahiti. Antedon. West Indies to Brazil; Norway to the Gulf of Guinea; Mediter- ranean. Bay of Bengal and the antarctic legions south of the Indian Ocean to the Hawaiian and Galapagos Islands, the west coast of North .America, the Aleutian Islands and Japan, including the Sea of Japan. L eptometra. Madeira to the Hebrides; hlediter- rmean. b) Atlantic genera finding their nearest relatives in genera which do not occur west of the eastern part of the Bay of Bengsl. Eastern part of the Bay of Bengal eastward to the mid-Pacific be- tween Oceania and America. Bathycrinus. Mid-equatorial Atlantic northwest- ward to the coast of Maxyland. Intern. Rev. Biol. Ruppl. z.VI. Bcl. 7 10 Austin H. Clark.

Bay of Bengal and the antarctic regions south of the Indian Ocean to the Sunda, Philippine and Hawaiian Islands. Trich o met r a. Caribbean Sea to the Grand Banks of Newfoundland; Bay of Bis- cay to Ireland. Ha thromet r a. Arctic Ocean and southward to Chesapeake Bay and Portugal; antarctic regions south of the Indian Ocean. Bay of Bengal and the vicinity of Banda and Celebes. Mo n ach o crinus. Caribbean Sea to the Azores and Morocco, andnorthwardto south- west of Iceland. 111. Atlantic genera finding their nearest relatives in genera oc- curring in the extreme south of the Indian Ocean and in the East Indies. Antarctic regions south of the Indian Ocean and Australia to the Meangis and Hawaiian Islands and Japan. Th auma t o cr inus. Southwest of Iceland. Extreme south of the Indian Ocean to the Hawaiian Islands. Zenometr a. Caribbean Sea. IV. Atlantic genera finding their nearest relatives in genera oc- curring in the East Indies, but no further west or south. Admiralty Islands to Japan. Co ccom e tra. Ckibbean Sea. Sunda Islands and the Malay Archipelago to the Hawaiian Islands. Ate le crinu s. Caribbean Sea and southward .to Brazil; west coast of Ireland. Sunda, Philippine, Meangis and Kermadec Islands. Endoxocrinus. Caribbean Sea and southward to Brazil; from south of the Canary Islands north to Rochefort. Sulu Archipelago; Sunda Islands; Moluccas. Democrinus. Caribbean Sea; coast of Morocco. Celebes. G ephyr o crinus. Canaries and Madeira. Ki Islands. Adelomet r a. Cuba. The Atlantic Ocean Biologically an Inland Sea. 11 V. Atlantic genera finding their nearest relatives in genera oc- curring in the Pacific Ocean, but not in the East Indies or further west. B athg m e t r a. Off the Abrolhos Islands, Brazil. VI. Atlantic genera finding their nearest relatives in genera occur- ring in Australia, and thence northward and westward into the adjacent portions of the Malay Archipelago. Australia north to the Philippine Islands, Hong Kong and Singapore, and west to the Andaman Islands. Co m acti ni a. Southeastern United States and southward to Brazil. Southeastern Australia northward and westward to the Andaman Islands. Ana1 ci d o ni e t r a. Caribbean Sea. VII. Atlantic genera with no close relatives in the indo-Pacific region. Is ometra. Off Montevideo and south to the Burdwood Bank. R hiz o crinus. North Atlantic from Florida and southern Ireland northward. Holopus. Caribbean Sea.

The preceding tables may be condensed as follows: I. Atlantic genera the closest relatives of which range from east Africa to the East Indies. a) These related genera occur in the Arabian Sea. Crinometra. Am. Pentame t ro crinus. Am.; ELI. Heliometra. Am.; Eu.; Arctic. Bythocrinus. Am.; Eu. b) These related genera occur in southeastern Africa. Stylometra. Am. Nemaster. Am. Neocomatella. Am.; Eu. Hypalometra. Am. Crotalometra. Am.; Eu. c) These related genera occur from the Arabian Sea southward to southeastern Africa. Tro piometr a. Am. Leptonemaster. Am. - Comatilia. Am. 13 Austin H. Clark. d) These related genera occur from the Arabian Sea to the Antarctic. Thalassometra. So. Atl.; Eu. Thaumntonietra. Am. 11. Atlantic genera the closest relatives of which do not occur west of the Bay of Bengal. a) These related genera occur at Ceylon. Antedon. Am.; Eu.; Med. Leptometra. Eu.; Med. b) These related genera do not occur west of the eastern part of the Bay of Bengal. Bathycrinus. Mid-Atl.; Am. Hathrometra. Am.; I~u.;Arctic. Trichometra. Am.; Eu. Monachocrinus. Am.; Eu. 111. Atlantic genera the closest relatives of which do not occur west of the East Indies except in the extreme south of the Indian Ocean. Thauma tocrin us. Am. Zenometra. Am. IV. Atlantic genera the closest relatives of which do not occur west of the East Indies. Coccometra. Am. Democrinus. Am.; Eu. Atelecrinus. Am.; Eu. G e p h y r o c r inu s. E 11. Endosocrinus. Am.; Eu. Adelometra. Am. V. Atlantic genera the closest relatives of which occur in the Pacific Ocean, but not in the East Indies nor further west. Bathymetra. Am.

VI. Atlantic genera the closest relatives oE which occur in Australia, and in the immediately adjacent portions of the East Indies. Comactinia. Am. An a1ci d o m e tr a. Am.

The first point to be emphasized in the discussion of the crinoids of the Atlantic is that none of the genera represented can be assumed to be of ihtlanticlorigin. Many of the genera occur only in the Atlantic but, except for three, these are all very closely related to Indo-Pacific genera of which they dre the (almost invariably more generalized) Atlantic representatives. The three exceptions, genera ,which have no representatives in the Indo-Pacific region, are Isocrinus, Rhizocrinus and Holopus. The Atlantic Ocean Biologically an Inland Sea. 13 But I s o c r i nus is found in the Trias and Jurassic of North America, Europe, the Moluccas and Australia, R hi z o c rinu s occurs in the Cretaceous of New Jersey as well as in the Eocene of Europe, and Holopus occurs in the Tertiary of Italy, so that none of the three can be logically supposed to have originated as Atlantic genera. In the Indo-Pacific region there are no less than fifty genera (two thirds again as many as the entire number inhabiting the Atlantic and Arctic Oceans together) which are not in any way related to Atlantic types; furthermore, eight families out of pineteen and one of the subfamilies of another (the subfamily Comasterinae of the Comasteridae) are exclusively confined to this region. Of the 31 genera known from the Atlantic 16 occur only on the American side, 3 only on the European side, while 12 are common to the two sides. Of the 14 Atlantic genera represented by closely related genera occurring on the coast of east Africa 8 occur only on the American side, 1 only on the ,European side, and 5 are common to both sides. Of the 17 Atlantic genera represented in the Indo-Pacific region by related genera \which occur no further west than the Bay of Bengal 8 arc exclusively American, 2 are exclusively European and 7 are common to both coasts. Of the Atlantic genera represented by closely allied genera only in the Pacific Ocean (east of the East Indies) and about Australia and the immediately adjacent islands all 3 are exclusively American. Considering only the 12 !Atlantic genera the Indo-Pacific represen- tatives of which occur in water of less than 50 fathoms in depth (Ne o c om a tel la, Nem a s t e r, C o m act i n ia, Lepton e m a s t er, Coma t i 1ia, T r o p i om e t r a, A n a1 c i dome t r a, ,C rot a1 o - metra, Crinometra, Antedon, Coccometra and Democri- nus), we find that of these representatives occurring on the east coast of Africa 5 are the equivalent of American genera exclusively, while 2 are ihe equivalent of genera occurring on both coasts; of those occurring only from the Bay of Bengal eastward 3 represent American genera and 2 genera common to both coasts; altogether 8 oE these genera represent exclusively American types and 4 repre- sent types common to both coasts. Considering only the 19 Atlantic genera the Indo-Pacific represen- tatives of which occur in water of more than 50 fathoms in depth (Stylometra, Thalassometra, Zenometra, Adelometra, 14 Austin H. Clark L e p t o m e t r a, H y p a1 om e t r a, H e 1i o met r a, Hathr o m e t r a, Trichometra, Thaumatometra, Bathymetra, Thaumato- crinus, Pentametrocrinus, Atelecrinus, Endoxocrinus, B ath y cri nu s, Mona c h o cr i nu s, By tho cri nus and Ge p h y- r o c r i n u s) 8 are exclusively American, 3 are exclusively European, and 8 occur on both coasts. Considering only the 10 Atlantic genera which occur in less than 50 fathoms (Nemaster, Leptonemaster, Tropiometra, Lep- tome tra, Anted on, H eliom e t ra, Ha throme t r a, C o ccome- tra, Endoxacrinus and Democrinus) we find that of the Indo- Pacific representatives 4 reach east Africa, 3 reach the Bay of Ben- gal, while 3 occur only in the East Indies; of the 4 reaching east Africa 3 represent American types and 1 represents a type common to both coasts; of the 3 reaching the Bay of Bengal I represents a European type and 2 represent types common to both coasts; of 'the 3 in the East Indies 1 represents an American type and 2 repre- sent types common to both coasts. One of the most striking points brought out by the preceding ana- lysis of the Atlantic crinoid fauna is the great richness of the Ameri- can as compared with the European side of that Ocean. On the American shores there are no less than 28 genera represented, as against 15, only about half as many, on the European; while there are 16 genera peculiar to the American side, there are only 3 con- fined to the European side. This greater richness of the American side of the Atlantic, which is almost entirely localized in the Caribbean Sea, probably is the result of two factors, (1) the Caribbean region was connected with the East Indian region in some way more closely and longer than any part of the European or African coast, and (2) the West Indian archi- pelago, being greatly diversified, offers a much better home €or crinoids, so that types may persist there which have died out every- where else. Several genera represented by littoral and sublittoral types in the East Indies occur in water of moderate depth in the West-Indies, but in much deeper water off northwestern Africa and southwestern Europe, where the oiily littoral spicies are those belonging to the genus Antedon. Probably in the east Atlantic these forms are unable to find sufficient or suitable food in the water of the surface currents, and therefore are restricted to the deeper waters, which The Atlantic Ocean Biologically an Inland Sea. 15 are constantly carrying northward a supply of minute antarctic organisms . Another interesting point is that mlany of the Caribbean crinoid types which occur in deep water among the Lesser Antilles range far to the southward in shallower water, while on the opposite shore similar types have extended their distribution in exactly the opposite direction, ranging from the coast of Morocco far to the northward iand northwestward. This extension southward of the species of the western Atlantic, and the corresponding extension northward of the species of the eastern Atlantic, is the result of the action of the deep water currents along the two coasts, and has no further significance. Since the Atlantic Ocean possesses no crinoid types not identical with or closely related to types in the Pacific and Indian Oceans, just as the Mediterranean Sea contains no types not closely related to Atlantic forms, the Sea of Japan none not clmely allied with others in the Pacific or Arctic Oceans, the Sea of Okhotsk none not existing in the Pacific or Arctic Oceans, and the Bering Sea none not ocicurring along the Pacific coast of North America to the east and south, it would seem that faunally the Atlantic Ocean is an inland sea of the Indo-Pacific Ocean just as truly as the Mediterranean is an inland sea of the Atlantic, and the Japan, Okhotsk and Bering Seas are in- land seas of the Pacific, even through it is Tar richer than those seas in endemic species. The Sea of Japan has received its crinoid fauna in two ways; those of the eastern side eiitered through the Korean Straits with the warm water of the Kuro-Siwo, while those of the cold water along the western shore entered with the cold current from the Sea of Okhotsk. We only know from the Sea of Okhotsk the widely distributed Arctic genus Heliometra, and one or two forms which have pene- trated into the warm water in the vicinity of the Straits of La PBrouse with the warm water from the Sea of Japan. From the Bering Sea we only know crinoids which have crept in from the Pacific coast of Alaska and North America south of Alaska; but the belt of cold water along the Kamchatkan coast has not as yet been thoroughly explored. It is quite probable that there will be found here the arctic species characteristic of the western side of the Sea of Okhotsk and of Sea of Japan. 16 Austin H. Clark. In the Mediterranean there exist two genera, Antedon and L e p tome t r a, the former occurring also on both sides of the ht- lantic, the latter on the Atlantic coasts of northwestern Africa and Europe; both are most closely related to other genera occurring in the Bay of Bengal and further east. This last is an important point, for it suggests that once the Mediterranean bore the same relation to the eastern Indian land Pakific Oceans on the one hand and to Ihe At- lantic on the other that the Sea of Japan now bears between the Pa- cific Ocean (as Irepresented By the Eastern Sea) on the south and the Sea of Okhotsk on the north; in other words, that once it served as a cianal through which crinoids migrated from the region of the Bay of Bengal to the Atlantic. Faunal conditions in the Atlantic are far more complicated than in the Mediterranean, the Bering, the Okhotsk or the Japanese seas. All the elements of the fauna of the Sea of Japan entered that basin by means of two routes; the species of the eastern coast, living in warm water, entered through the Straits 'of Korea, those of the western coast, living in cold water, entered from the Sea of Okhotsk through the Straits of La Pkrouse. So far as we know exactly the same is true in the Sea of Okhotsk; the species of the warmer water entered through the Straits of La Pkrouse, those of the colder water with the Bamchatkan current. In the Bering Sea all the species which we know entered with a cold deep current from the Gulf of Alaska. But in order to explain satisfactorily the conditions in the Atlantic it is necessary to assume that there have been no less than €our routes by which crinoids have entered that basin from the east, two of which, the two most im'portant, are now closed. These four routes are as follows: 1. From the Arctic. -- The few crinoids which have intruded into the Atlantic from the Arctic are all inhabitants of very cold water, and they are more or less sharply differentiated in habitat from the crinoids which have entered by different routes through a more or less distinct temperature barrier, as they live only in very cold water. The Arctic fauna inself appears to be a derivative from the fauna of the Indian Ocean as represented in the Bay of Bengal. Outside of the Arctic Ocean it occurs only in the Sea of Okhotsk and in the Sea of Japan, where it is characteristic of the belt of very cold water along their western shores. The Atlantic Ocean Biologically an Inland Sea 17

2. From the extreme south of the Indian Ocean. - The deep water crinoids of the Atlantic are all representives of types characteristic of the deep water of the Indian and Pacific Oceans and are very closely allied to the species of the wdters about southern Africa and further southward; these types do not occur in the Magel- lanic region. Properly speaking this route is in reality only a part of the suc- ceeding (3), including the deeper water species characteristic of it; but as it is still open to migration while the succeeding has long been closed it seems best to keep them separate. 3. From the southeastern Indian Ocean (the Madagas- can region). - This path of migration, now closed, extended from the region of Madagascar in a general northwesterly direction to the Antillean region. It was by this path that many crinoids reached the Caribbean region which were never able to establish themselves on the eastern coasts of the Atlantic. The faunal affinities of the Madagascan region are with the Lesser Sunda Islands and Australia, and there are very strong indications that the connection between the Antillean region and Madagascar was continued eastward across what is now the Indian Ocean to the Australian region. The disruption of the part of this connection crossing the Atlantic, followed later by the disruption of the portion crossing the Indian Ocean, served to isolate completely the Antil- lean region, and also to isolate to a very considerable extent the Australian region. The Madagascan region, being more exposed to the intrusion of types originating subsequent to the disruption of this connection, has had its fauna more modified than the fauna of the Antilles or of Australia. Thus it is that in the Antilles we find the genus C om acti nia, the nearest relative of which is the Austra- liah Comatula, and Analcidometra, most closely related to the Australian 0 1i g o m e t ri d e s. 4. Through the Mediterranean region. - At the time of

, the passage of crinoids from the Indo-Pacific region into the Atlantic by way of the Mediterranean, that sea was of far greater extent than at present, and reached eastward to the Bay of Bengal. This path of migration has long been closed; only two genera of the many which reached Europe by this route now survive, both of them being but sligh'tly differentiated from the original Indo- Pacific types from which they took their origin. Of these two genera 18 Austin H. Clark, The Atlantic Ocean Biologically an Inland Sea. one includes only littoral species, just as do its eastern allies, while the other includes species inhabiting water of moderate depth, it, too, corresponding in this respect to its Indo-Pacific representative. The former (Antedon) is more plastic than the latter and, though its species are strictly littoral, they have collectively a greater range than the species of the latter (Leptometra). Thus we are justified in announcing that, so far as the evidence to be deduced from the study of the recent crinoids permits us to judge, the Atlantic Ocean is biologically, as it is geologically and oceanographically, an inland sea, for it has received its entire fauna from outside, by means of four different routes, only two of which are now open, and has never originated any type within its own basin. Furthermore it is an inland sea largely, if not entirely, formed through the unequal subsidence of a land mass; for the fauna of the Caribbean region, composed of ancient and generalized types, appears to have become established before the intrusion of the spe- cies from the Mediterranean region, which are more modern and more specialized, became possible.