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Show 1899.] CRUSTACEAN FROM LAKE TANGANYIKA. 703 much suggests a distortion produced by the frontal downgrowth, also supports this view. The greater number of spines occurring on the antero-lateral margins is a further feature, capable, however, of two possible interpretations. The carapace in but few species of Thelphusa bears more than one, and that a less prominent spine. If, then, we are dealing with a multiplication of marginal spines, we have an indication of greater specialization than that met with in Thelphusa, an indication contrary to the tendency of the other evidence. The other possible explanation, then, that a reduction towards the extreme condition of Thelphusa is in progress, would seem far more probable ; and it is a noticeable fact that the marine and littoral Crabs, from which we may suppose this form has been derived by comparatively slight modifications, are far more spinous than auy of the modern terrestrial or fluviatile forms. Thus, while Platythelphusa and Limnothelphusa would appear to be the most primitive of these Old World genera, Parathelphusa, in the less pronounced arching of the carapace and the more numerous lateral spines, would come as intermediate between them and the most specialized condition of Thelphusa. O n the causes which have contributed to the present-day distribution of these genera, a word or two may be said. It is no very recent conception that Madagascar aud, through this island, the south of Africa itself, was perhaps at some remote period connected in a tolerably close manner with India. The present fauna of Madagascar, which shows marked Oriental affinities, bears this out; aud from considerations of geological facts, particularly as regards the possession of a common flora in Carboniferous times, Dr. Blanford \ following Suess and Neumayr, is inclined to regard the idea of a great continent, embracing Australia, India, and South Africa, as by no means improbable. The evidence for such a land-connection is not confined to beds of quite such ancient date, however, for both in Jurassic and Cretaceous times the fauna of the two areas is distinctly suggestive of this same continuity. If, then, we may imagine the ancestral Thelphusa as living on the shores of this early continent, in which the present Lake Tanganyika was represented as a narrow bay or fiord, it is not unreasonable to suppose that while Limnothelphusa, and perhaps Platythelphusa, staying in the lake, retained most nearly the ancestral characters, Hydrothelphusa and Parathelphusa2, still largely aquatic in habit, would resemble them more nearly than Thelphusa, many species of which spend most of their time upon land. It is of course difficult to tell how far auy one character, or even collections of characters, may be primitive or adaptive, or again, whether an intermediate stage of greater specialization might not be attained and lost again on change of surroundings. On the whole, however, I conclude that this Crab presents rather lowly characters in the group to which it belongs. 1 Anniversary Address to the Geological Society, 1890. 2 Wood-Mason, loc. cit. p. 122. |