The Molluscs of the Great African Lakes. I. Distribution. by J

THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 159 The Molluscs of the Great African Lakes. I. Distribution. By J. £. S. Moore. THE present paper forms the first instalment of the zoological report of an expedition which, through the generous support of the Royal Society and the British Association, I was able to make to Lake Tanganyika during 1895 and 1896. The primary objects of this expedition were— 1. To study the unique fauna of Lake Tanganyika on the spot; 2. To make what observations were possible in the Nyassa region while I was en route; and— 3. To bring back properly preserved material for the complete morphological investigation of the more remarkable lake organisms after I returned. Before proceeding to the purely zoological matters with which I propose to deal, it is appropriate that I should here express my sincere thanks to Professor Ray Lankester, to whom I have been indebted for the primary suggestion of the whole inquiry, and for much kindly help since my return. I have also to thank Professor G. B. Howes for the use of the Huxley Laboratory and invaluable advice, without both of which I should never have been able to get the subject through, while I have been very materially indebted to Sir John Kirk, who procured for the expedition the necessary introductions to the administrative gentlemen through whose districts it had to pass. And last, but not least, I have" to 160 J. B. S. MOOIUB. thank Sir Harry Johnston for the very effective support he lent the expedition, and without which it would have been impossible for me to attain the objects which I had in view. Excluding the polar regions proper, there exists in the fresh waters of the different continents a type of fauna which in the character of its constituents is essentially the same. Certain forms are added and others are omitted as we pass from the more temperate to the equatorial zones, but beneath these changes there exists a substantial similarity, so easily recognisable and so marked that geologists have not hesitated to distinguish between fresh-water and marine fossiliferous deposits wherever they may be found. On the other hand, that there is a hard and fast demarcation between fresh- water and marine faunas is not true, for there are many instances of animals—for example, of prawns and crabs, which in this country are purely oceanic—having made their way up the rivers into inland and elevated fresh-water lakes. Further, there are a number of animals that belong neither to salt nor fresh water, but are inhabitants of the brackish regions which lie between inland fresh waters and the sea. That the well- established and more permanent fresh-water organisms of the present day are descended from older phyla that were once marine, is an accepted truth. This view is necessitated by the theory of common descent, and it is supported, as in the case of the ganoid fishes, by the similarity of numerous living fresh-water organisms to older oceanic types. It is sig- nificant, however, that with few or no exceptions, all the well- established fresh-water organisms of to-day are not directly referable to the earliest oceanic forms, but rather to those which in their temporal distribution stand intermediate between then and now. It seems that the fresh-water molluscs of the present day first make their definite appearance in Tertiary times, for much doubt has recently been raised as to the genuineness of the so-called carboniferous Unio, Ticho- gonia, and Planorbis; these forms being now regarded as more nearly related to Anthracosia, Avicula, and THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 161 Serpula respectively. The family Limnseidse, which is now so universal in its distribution, does not certainly extend further back than the Jurassic period. The same is true of the fresh-water Melaniidse and of the Paludinidse, but I need scarcely point out that it is necessary to use the greatest caution in drawing any inferences respecting the date of origin of the true fresh-water forms from these apparent facts. It may, however, be taken as approximating to the truth to say that although the typical and universal fresh-water molluscs of the present do not appear upon the stage of life as such before the Jurassic period, they almost certainly origi- nated from a series of marine types which had become com- pletely differentiated from their oceanic associates long before this time. Some of these antecedent organisms are probably represented in the palseontological record by those extinct genera with which the earliest known modern fresh-water types are usually associated.1 The facts of morphology are themselves in harmony with such a view, for in their anatomy the living fresh-water molluscs do not approximate to any of the more modern marine genera; they hark back to those more permanent marine types which were in existence long before Jurassic times. They certainly bear no resemblance to the generalised conceptions or archetypes of the more modern marine genera which appeared during Tertiary and post-Tertiary times, such as Strombus, Pteroceros, Rostellaria, Conus, Mitra, Chenopus, and the like. It is this fact that is of first importance to us here ; for if it should be found that in some district at the present time there exists a fresh-water fauna which departs from the normal and universal type in the possession of genera which approximate to those that are undoubtedly modern and marine, we shall have very strong prima facie evidence for regarding these organisms as recent 1 II; is quite possible that many of the old so-called fresh-water deposits are in reality marine, since the forms which became exclusively fresh water as time went On probably made their appearance in the sea first, as so many of the more recently derived fresh-water types have done,—prawns, for example. VOL. 41, PART 1. NEW SERIES. 1 162 J. E. S. MOORE. importations from the sea. Now such a fauna is presented to us in that of Tanganyika at the present day, for in this lake there have been known to exist ever since 1859 what appear to be the shells of some six genera of Gastropods, which are entirely unlike any known fresh-water forms, while their shells at the same time simulate several modern oceanic types. The interest in these strange molluscs, which have been known hitherto only by their conchological characters, was greatly augmented when in 1883 Boehm found jelly-fish in the lake; and during my recent expedition I have been able to add deep-water crabs, prawns, sponges,1 and Protozoa to this anomalous list of organisms, all of which appear to possess the same marine affinities. It is my object in the present paper, to ascertain to what conclusions as to the nature and origin of this anomalous series the facts of distribution lead ; while in those which follow I shall deal with the morphological affinities of the hitherto unknown individual forms, and thus determine whether the conclusions to which these facts of distribution seem to point are really sound. Almost no definite observations have been hitherto available for the study of this subject, and consequently the material contained in this and the following papers will be mostly new. I would, therefore, invite particular attention to the positive character of the evidence which I shall bring forward in support of the recent marine origin of a number of the animals contained in Tanganyika, as compared with the wholly negative character of that upon which the geological speculations of Murchison respecting the " permanence of terrestrial conditions " in the African interior at present rest. To believe that the marine animals of Tanganyika are among the few remaining indications of a sea that once extended to the very heart of the African continent is to come into the most uncompromising conflict with the theory put forward by 1 The affinities of the deep-water sponge which I obtained in Tanganyika have not yet been determined, but its striking external character and its remarkable deep-water habitat have inclined me to regard it as a member of the anomalous section of the fauna which the lake presents. THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 163 Sir Roderick Murchison in 1852. Yet such a view is now supported by the strongest kind of zoological evidence it is possible to get. In order to arrive at a sufficiently complete comprehension of the general type of the molluscan fauna which characterises the individual lakes, it is quite unnecessary and even prejudicial to discuss the question of the occurrence or non-occurrence of many of the so-called specific forms, since a large number of these are merely geographical varieties, while others have been based on minute and often purely fanciful conchological distinctions. The four species of Hylacantha, described by Bourguignat in 1890, for example, are certainly nothing more than the rather remarkable polymorphs of the original Typhobia Horei described by Smith in 1881. But what is true of the polymorphic Typhobia is equally true of the polymorphic Paramelania and Neothauma. I shall therefore consider the distribution of the genera alone, or the main issue will become lost in the pursuit of really non-existent types. In Nyassa, which was the first great lake I reached, and which has hitherto been better known than all the rest, there have been recorded some sixteen genera of molluscs, namely, Limn tea, Isodora,, Physa, Physopsis, Planorbis, Ancylus, Ampullaria, Lanistes, Vivipara, Cleopatra, Bythinia, Melania, Spatha, Iridina, Corbicula, and Unio.

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