August 25, 189SJ 



NA TURB 



405 



indicate how mucli .-urprisi tlie discovery of jelly-f.sh in Like 

 Tanganyika, by Dr. Boehm, created in the minds of thdse who 

 were interested in the past history of the great lakes in Africa, 

 for, in the presence there even of a single organism so typically 

 marine, and so unlike any real fresh-water form as a medusa, 

 there was as good, indeed far better, evidence for the former 

 access of the sea to those regions, than that which was afforded 

 by the herrings in the Dutchman's ditch. 



It was i>artly because I held this view, in regard to the 

 presence of jelly-fish in Tanganyika, more especially because 

 Prof. Lankester pointed cut to me that where there were jelly- 

 fish one might reasonably expect to find other marine organisms, 

 similarly cut off, that I went to Tanganyika in iSgi;. The 

 results of that expedition have fully justified these views, and 

 during the past year, in which the zoological material obtained 

 has gradually been overhauled, it has become more and more 

 apparent that in Tanganyika we have not only a jelly-fish, but 

 the remains of an entire fauna, which can be regarded as 

 nothing but the relic of the former extension of some ancient sea. 



Thus besides the jelly-fish there exist on the rocks about the 

 shores, and in the deep water of the lake, numbers of molluscs, 

 which not only in their shell structure, but also in their 

 organisation, show clearly that they belong to those groups 

 which have generally remained marine, and which have never 

 given rise to any of the colonising fresh-water types. Besides 

 these there are at least two forms of prawns, a deep-water crab, 

 and several forms of protozoa, all possessing like marine affinities. 



At the same time it is most important to remember that 

 Tanganyika contains its full complement of recognised fresh- 

 water forms, which are similar to those constituting the entire 

 fauna of lakes such as Nyassa, Mwero, and the like, and that 

 these fresh-water types in Tanganyika differ from those in Lake 

 Mwero and Nyassa only to the same extent that those in Lakes 

 Mwero and Nyassa differ from each other. It is thus obvious, 

 and one of the most important results hitherto obtained, that 

 the fauna of Lake Tanganyika is to be regarded as a double 

 series, one half consisting of forms which are found everywhere 

 in the African fresh waters, the other of what we may call 

 halolimnic organisms, which are found living nowhere else in 

 the world, at least so far as is at present known.' 



In the incomplete state of our knowledge of the Halolimnic 

 fauna, it is undoubtedly the mollusca belonging to this group, 

 which are the most instructive at the present time ; for among 

 these organisms there are a considerable number of types which 

 are widely different from each other, and all of which can be 

 compared with living oceanic forms. We have here, therefore, 

 a basis of comparison broad enough to give a clear and trust- 

 worthy conception of their nature and their actual affinities. 



In this way it is clearly seen that in several genera of the 

 Halolimnic mollu-^cs, such as Typhobia, Bathanaiiu, and others, 

 we have forms which individually do not correspond exactly to 

 any single living oceanic species, but which at the same time, 

 in the curious character of their organisation, do very distinctly 

 foreshadow and combine the anatomical features not of one, but 

 of several living oceanic species which are now quite distinct 

 from one another. The only conclusion, therefore, that can be 

 drawn from this remarkable character of the Halolimnic forms, 

 is that they have been cut off approximately all at the same 

 lime from their original marine associates at an extremely 

 ancient date. In fact, that they still retain combined the 

 original characters of the organisms whose progeny in the ocean 

 has become completely differentiated into forms that are now 

 specifically and even generically distinct. 



These Halolimnic molluscs stand, therefore, to such oceanic 

 species in the relation of ancestral types. 



This inference respecting the great antiquity of the marine 

 fauna in Tanganyika, which we gather from the peculiarities of 

 the organisation of the individual Halolimnic forms, is in exact 

 accord with what we should expect when contemplating the 

 vast physical changes which must have been produced since 

 there was any possibility of Lake Tanganyika communicating 

 freely with the sea. But although from both these sources of 

 evidence we are assured that the Halolimnic fauna is certainly 

 a " hoary relic" of the past, they are neither of them capable of 

 affording any indication of the particular geological period 

 during which the marine contamination of this part of the 

 African interior actually took place. 



Quite recently, however, there has come to hand a series of 



1 See my papers, /*r<>c. Koy. Soc.,yo\. Ixii., 1898, pp. 451-458; and 

 Quart. Joum. Micr. Sci., vol. xli. pp. 159-180. 



NO. 1504, VOL. 58] 



observations which appear to be of the highest interest in this 

 connection, and capable of throwing a considerable amount of 

 light upon the perplexing question of the relative antiquity of 

 the Halolimnic forms. It has been found, after comparing the 

 peculiar shells of many of the Halolimnic molluscs, such as those 

 of the two forms of Limnotrochtis, the genus Bathanalia, Spekia, 

 Paramelania, and so forth, with the fossilised remains of the 

 molluscs occurring in successive geological periods, that there 

 exists a wonderful similarity between the general facies of the 

 shells belonging to the marine fauna of Lake Tanganyika and 

 those of the old Jurassic seas. This is no merely superficial 

 resemblance between single types, but a substantial conchological 

 identity between so many Halolimnic genera and species and an 

 equal number of forms occurring in the Lias and Inferior Oolitic 

 rocks, that it at once arrests attention, and requires us to con- 

 sider very carefully, whether we are to regard this similarity of 

 the two series as merely a coincidence, or the expression of some 

 real community of nature and descent. 



Without entering too fully into the details of this subject, it 

 may be stated, as the result of a careful comparison of these 

 forms, which will be found fully described in a paper in the 

 Quart. Joiirn. Micro. Sci., vol. xli. No. 162, June 1898, that 

 the comparison is so striking and so complete in detail, that had 

 the Halolimnic molluscs been known only in some fossiliferous 

 bed, there is not the slightest doubt that even the most fastidious 

 palaeontologist, unless he had a particular theory to support, 

 would regard them as unquestionably belonging to Jurassic seas. 



Taking, therefore, a retrospective view of the whole matter, 

 it will be seen that the original discovery of jelly-fish in Tangan- 

 yika has led us a long way beyond the mere demonstration of 

 the existence of a marine animal in the African interior. It has 

 brought to light the existence of a long series of other marine 

 organisms, which, judged by the nature of their organisation, 

 are unquestionably very old, while, finally, we have obtained, 

 evidence which appears to indicate that, at any rate, the mol- 

 luscs still living in this marine oasis in " terra firma," are relics 

 from Jurassic seas. 



Thus the purely scientific interest of the Halolimnic fauna 

 consists mainly in the way in which the different forms com- 

 posing it afford an insight into the structural peculiarities of a 

 number of types of organisation which were thought to have long 

 since become extinct ; but at the same time the presence of this 

 fauna in Tanganyika is destined to throw a world of light on 

 the past history of the continent in which it lives, and it is all 

 the more interesting in this latter sense, because the past history 

 of the African lakes, as read in the light of the Halolimnic group, 

 is not that which many geologists, particularly Sir Roderick 

 Murchison, have supposed it to have been. 



I have thus briefly outlined the extent and nature of the latest 

 information which has been acquired respecting the zooK>gy of 

 the African lake districts, and the extent to which these observ- 

 ations may change existing preconceptions, and throw old pro- 

 blems into new perspective, will constitute their value from a 

 philosophic point of view. But for the practical ends and ad- 

 vancement of zoology, it will be obvious that the conclusions 

 which have been attained respecting the vast antiquity of the 

 Halolimnic forms, foreshadow the possibilities of almost infinite 

 developments, and that the value of further exploration of these 

 lakes, as a zoological speculation, has become immense. 



It is therefore greatly to be regretted that during my recent 

 expedition, under the circumstances in which I found mysel. 

 (without a steamer, and consequently unable to use deep-water 

 dreging apparatus), it was quite impossible to form even an 

 approximate estimate of the range of animals one might expect 

 to encounter in the Tanganyika, and more exasperating than 

 this was the fact that the most interesting Halolimnic forms, the 

 Typhobias, Bathanalias, and their associates, only appeared just 

 at the limit of my dredging powers, about 1 000 to 1200 feet. 

 It was thus only when the dredging capacities of the expedition, 

 so to speak, were giving out, that the more interesting repre- 

 sentatives of the Halolimnic fauna were beginning to come in, 

 and there is no doubt that with a steamer and efficient apparatus 

 for great depths, many entirely new forms would be obtained. 

 To show how incomplete our knowledge of the fauna of Lake 

 Tanganyika at present really is, it may be pointed out that 

 although twenty-eight entirely new species of fish were obtained 

 during my expedition, of the four species previously known from 

 this lake I only re-discovered one (see Appendix). 



It should, however, be clearly understood that the zoological 

 and geological interest which the possible existence of new 



