1899.] CRITSTACEAiS' FROM LAKE TANGANYIKA. 703 



much suggests a distortion proclucetl by the frontal downgrowth, 

 also supports this view. The greater number of spines occurring 

 on the antero-lateral margins is a further feature, capable, how- 

 ever, of two possible interpretations. The carapace in but few 

 species of Thelphxisa 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 The^phuaa 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 any of the modern terrestrial or fluviatile 

 forms. Thus, while Platythelphusa and Lhnnothelpihusa would 

 appear to be the most primitive of these Old World genera, 

 Paratliclplmsa, in the less pronounced arching of the carapace and 

 the more numerous lateral spines, would come as intermediate 

 between them aiid the most specialized condition of Thelphusa. 



On the causes which have contributed to the present-day dis- 

 tribution of these genera, a word or two may be said. It is no 

 very recent conception that Madagascar and, 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 ; and from considerations of geological facts, par- 

 ticularly as regards the possession of a common flora in Carbo- 

 niferous times, Dr. Blanford S 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, HydrotheJp)husa and Parathelphusa ^, 

 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 any 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 lowlv 

 characters in the group to which it belongs. 



' Anniversary Address to the Q-eological Society, 1890. 

 * Wood-Mason, loc. cit. p. 122. 



