274 DR. W. A. CUNNINGTON ON THE BRACHYUROUS [Mar. 5, 



P. maculatcc, and dredged among shells in 10-15 fathoms of water 

 at the south end of the lake. 



3. General Hemarks. 



The principal result of our extended knowledge of the Brachyura 

 of the African lakes is to make still more clear the very special 

 nature of the Crab-fauna of Tanganyika. As is found to be the 

 case in so many different groups of animals, the forms occurring 

 in Tanganyika are for the most part endemic, while those found 

 in the other big lakes are often of wide distribution. There occur 

 in Nyasa, as we have seen, three species of Potamo7iautes, one of 

 which, it is true, is described from that lake only. In the Victoria 

 Nyanza, we find Potamon {Parathelplmsa) niloticiis, a very widely 

 distributed form, and P. (Geothelpkusa) emini, also known from 

 Abyssinia. From the Albert Edward ISTyanza there comes also a 

 species of Geothelpliusa. Thus, with the single exception of 

 P. i^Potamonautes) orbitospinus, from Nyasa, the forms at present 

 known from these big lakes are by no means confined to them. 



With Tanganyika it is quite otherwise. There are two species 

 of Pota'monautes, one of which is not known elsewhere, but beyond 

 this, three species of a unique and remarkable genus wholly re- 

 stricted to this lake. From the other lakes we have then only 

 representatives of the subgenera of Potamon — forms such as are 

 widely distributed in the tropical fresh- waters of the Old World. 

 From Tanganyika, while we have some representatives of these 

 normal African types, we have a preponderance of forms perfectly 

 distinct and occurring nowhere else. There is an indication, too, 

 of that richness of the Tanganyika fauna which is so noticeable 

 in some other animal groups. We know of five species of Crabs 

 from Tanganyika, three from Nyasa, and only two from the 

 Victoria Nyanza. 



It is necessary to add a few remarks on the affinities of the 

 genus Platythelphusa. The species P. armaia has been considered 

 to exhibit a distinctly marine appearance. Milne-Edwards stated 

 in the course of his original description that the Crab bore such a 

 resemblance to certain raarine or brackish- water Grapsidae that 

 we might relate it to that group, were it not for the development 

 of the abdomen and the absence of metamorphosis. In his book 

 on ' The Tanganyika Problem,' Moore * very rightly challenges 

 the value of such a character as the latter to the systematist. If 

 the absence of metamorphosis is the result of a particular habitat, 

 as we have reason to suppose, we ought not to take it into account 

 when we attempt to determine the affinities of a newly-discovered 

 specimen. At the same time it would seem as if the i-esemblance 

 to the Grapsidpe could be only a very superficial one, produced, 

 perhaps, by the more or less quadrilateral shape of the carapace. 



* Op. cit. p. 236. 



