1907.] CRUSTACEA OF THE THIRD TANGANYIKA EXPEDITION. 275 



It must be admitted that the considerable development of 

 spines, which we find in all these species, is a feature more 

 commonly found among marine Crabs than among those inhabiting 

 fresh-water, and so would in itself convey a false impression on a 

 casual inspection. But we need not go outside the subfamily of 

 the Potamoninse to find examples of Crabs which are just as 

 " marine looking," as far as the development of spines is concerned. 

 This is true of a number of species of Parathelphusa, and when 

 we pass to the other subfamilies of the Potamonidee, we have 

 Potamocarcinus, Valdivia, and Bilocarcimis, all extremely well 

 armed with spines. As with all these genera and subgenera, 

 there can be no doubt that Platythelphusa, despite its appearance, 

 finds its nearest allies in this group of forms which are essentially 

 and typically fresh- water in habitat. 



It is a matter of more difliculty to decide which of the allied 

 forms are most closely related to Platythelphii,sa. The species now 

 known as P. mactdata has been stated to be a primitive form *, 

 on account of the little deflection of the front, the nature of the 

 antennae, and the spine-bearing margins of the carapace. These 

 features are equally characteristic of the other two species of the 

 genus, and do seem to show less specialisation than the deflexed 

 front and inflated gill-chambers of some of the semi-terrestrial 

 species of the subgenus Potcmion. It is then not with the 

 latter, but near the genus Hyclrothel2}hnsa and the subgenus 

 ParailielpTiusa that we ought probably to place this distinctive 

 Tanganyikan genus. 



The problem of the origin of Lake Tanganyika, about which so 

 much has been said and written, is of course intimately connected 

 with the questions which have just been discussed. If the lake be 

 the modified remains of part of an ancient ocean, we may expect 

 its inhabitants to show both a primitive character and a marine 

 aspect. These, Mr. Moore considers, are exhibited by all the 

 members of his " halolimnic " group, among which are reckoned 

 the species of Platythelphusa. It has been shown, however, that 

 this genus has no better claims to a marine origin than other 

 representatives of the family, and at the same time, that while it 

 is not so specialised as certain allied forms which have adopted a 

 partially terrestrial mode of life, neither need it be considered as 

 the most primitive in the group. Still there is little evidence 

 that this form was ever anything but wholly aquatic, and it may 

 have become modified by the truly oceanic conditions prevailing 

 in Tanganyika, until it attained a superficial resemblance to 

 marine types. 



In his report on the Macrurous Crustacea of the Expedition, 

 Dr. Caiman shows t that they are to be regarded rather as 

 specialised than as primitive in character, and it may be asked 



* Proc. Zool. Soc. 1899, p. 702. 

 t Proc. Zool. Soc. 1906, p. 204. 



