558 DR. W. A. CUNNINGTON ON THE 



tsxhle are to be regarded as sjoecinieus wliicb, in the existing stjite 

 of our knowledge, it is impossible to identify. It is nevertheless 

 clear that the precise distiibution in the lakes of these repre- 

 sentatives of the subgenera of Potamon is of little, if any, 

 significance. 



In this connection it should perhaps be explained that the 

 grounds of identification of the river-crabs are unsatisfactory on 

 the whole, external characters of a compaiatively trivial and 

 fluctuating nature forming the basis of distinction. Systematists 

 are thus on less sure ground than in the case, for instance, of the 

 prawns, where a matter like the branchial formula aifoids more 

 satisfactoiy evidence of affinity. 



Tanganyika, with no less than three species of the endemic 

 genus Platythel2)husa, is evidently a case sui generis. There are 

 certain features in the anatomy of this unique genus which 

 suggest that it- is of a somewhat primitive and- unmodified 

 character, but at the same time nothing to indicate that it is 

 more definitely marine than the other members of the family to 

 which it belongs. Further, it is only to b.e regarded as unspecial- 

 ised in comparison with allied forms which have adopted a semi- 

 terrestrial mode of life, so tha,t it affords in reality no support 

 for Moore's view that Tanganyika is the altered remains of an 

 ancient sea. The genus with its three distinct species is rather 

 to be looked upon as one more example of variation a,nd 

 divergence, brought about, in all probability, by prolonged 

 isolation. Finally, it may be noted that this lake, with five 

 species, contains a larger number of difierent ci-abs than any of 

 the others under consideration. 



Those groups of smaller Crustacea which are often associated 

 under the heading Entomostraca, are (with the exception of the 

 marine Cirripedia) well represented in the African lakes. In- 

 formation concei'uing them is, however, for the most part, of 

 fairly recent date, since such organisms would be overlooked 

 by any but trained biologists, and the use of the tow-net in 

 Central Africa has even yet yielded results which are but 

 fragmentary and incomplete. All the earlier records are due 

 to the zeal and energy of Stuhlmann, and concern in particular 

 Lakes Victoria and Edward. In 1898-1900., extensive collections 

 were made in ISTyasa and its neighbourhood by Fiilleborn, and 

 this material, together with a supply from Victoria Nyanza 

 collected by Borgert, forms the basis of a comprehensive treatise 

 by Daday (76), in which the whole of the microfaiina is dealt 

 with. The collections made by the writer during the Third 

 Tanganyika Expedition have afforded information for the first 

 time concerning the Entomostraca of Tanganyika, as well as 

 providing additional I'ecoixls for Nya?a, and Lake Victoi-ia,. 

 Finally, the woi'k of Schubotz during the German Central Africa 

 Expedition has furnished fxu'ther particulars I'clating to Lakes 

 Kivu, Edward, and Albert. 



It may here be pointed out that these lower Crustacea, with 



