OCR Text |
Show 438 ON THE RESULTS OF THE TANGANYIKA EXPEDITION. [May 4, of molluscs of genera ali similar to those found in Nyasa. In the Victoria Nyanza, in Lake Baringo, and in several others it is the same. Thus it will be seen that Nyasa contains representatives of all the genera met with in the above enumerated lakes. Some of them contain more, some fewer, just as might have been anticipated from tbe sort of difference in their physiographical characters. Nyasa in fact contains a generalized series of the generic faunas of these other lakes, and an inspection of these forms is sufficient to show that they one and all belong to well-recognized lacustrine groups. If now, however, we turn our attention to Lake Tanganyika, we find not only the whole list of genera fully represented, but above and beyond this a number of quite new and strange forms : these constitute by themselves an isolated and an intensely interesting series, every member of which, wdth the probable exception of Neothauma, would, were not their habitat known, have been regarded without any hesitation as marine forms. But a somewhat closer examination of this group shows, in the first place, that the Mollusca themselves are not very similar to any forms now inhabiting the sea. Typhobia and certain other species seem to have certain points in common-their radulae, although differing from every known form, are like each other. The little Gastropod which bas been known as the Lythoglgphus of Tanganyika is certainly not related to that group, but approximates in anatomical characters as a whole to the group of the Para-melanlce. This last group is certainly very distinct from that formed by Ty phobia and its associates, but its members are quite as peculiar, isolated, and self-contained. A most remarkable variety of this group occurs in the deep waters of Tanganyika, where it is associated with Ty phobia, Llmnotrocus, and some new species. The true Paramelanice exist a little higher up, and the lesser forms of this species infest the barely submerged rocks. So far as I have yet been able to study the anatomy of these forms, they certainly seem to suggest a primitive, simple, or generalized condition of their parts. They do in fact bear much the same relation to the typical lacustrine and sea-molluscs that the Ganoids do to the typical freshwater and modern oceanic fishes. This incapacity they one and all exhibit of being associated directly with either the present lacustrine or oceanic faunas, seems to m e to point most strongly to their being the persistent or modified representatives of a sea-fauna which must have contaminated Tanganyika long ago. The similarity of various forms of Paramelania to certain fossil shells is a fact which has been already independently recognized by White and Tausch, who have called attention to the strong resemblance of these shells to the Pergullferce of the American and Southern European Chalk. I may here further point out how closely certain forms occurring in Tanganyika resemble the Purpurince, of the Inferior Oolite. The testimony of the geographical distribution of the fauna of the great lakes of Central Africa and of their morphological characters, so far as I have at the present time been able to go, seems |