204 DR. W. T. CALMAN OX THE MACRUROUS [Mar. 6, 



pp. 57-134, 1905) that certain species occasionally present 

 " mutations" leading at a single step from Caridina to Orimannia 

 and from Ortmannia to Atya, must be borne in mind as suggesting 

 the possibility that the Atya-Wke characters of Atyella may have 

 arisen independently in the Tanganyika forms. In any case, 

 there can be no'doubt that the Atyida? of Tanganyika rank among 

 the most highly specialised members of the family and are fai- 

 removed from such primitive forms as Xiphocaris and Atyaephyra. 



When describing the two species of Prawns discovered by 

 Mr. Moore in Tanganyika, I pointed out (Proc. Zool. Soc. 1899, 

 p. 711)* that they thi'ew no light on the general question of the 

 oris;in of the Tanganyika fauna, inasmuch as they belong to groups 

 which are characteristically inhabitants of fresh-water. Since 

 then, in his book on ' The Tanganyika Problem ' and elsewhere, 

 Mr. Moore has claimed that the prawns belong to the " relict," 

 or as lie terms it " halolimnic," section of the fauna of that lake. 

 He believes that the members of this section are distinguished by 

 special resemblances to marine forms and by generally primitive 

 chai'acters. He supposes that they i-epresent the descendants of 

 mai-ine species which reached theii' present habitat not later than 

 the Jurassic epoch, when the present site of the lake was occupied 

 by an arm of the sea. 



It is necessary, therefore, to state definitely that there is not 

 the smallest ground for svipposing that the Macrurous Crustacea 

 of Tanganyika have had such an origin. The groups to which 

 thev belong, the genus Palcemon and the family Atyidse, are 

 widely distributed in the fresh-waters of tropical regions, and 

 the fact that repi-esentatives of both occur in Tanganyika is, in 

 itself, no more surprising than the fact that i-epi'esentatives of 

 both occur in the Upper Nile. Nor is it the case that the 

 Tanganyikan species present such pl■imiti^■e characters as would 

 bring them closer to the hypothetical marine stocks from which 

 these groups have arisen. As regards the Atjddre, at all events, 

 the reverse is the case, for the Tanganyikan genera are in some 

 respects the most specialised members of the family. What does 

 distinguish the Macruran fauna of Tanganyika is the great 

 number of species found within a limited and continuous area t 

 and their distinctness, so far as we know, from all the species 

 inhabiting adjacent i-egions. The explanation of these peculiarities 

 is a very ditiicult problem and one which cannot be profitably 

 considei'ed apart from the similar problems presented by the other 

 elements of the Tanganyikan fauna. For the pi-esent, however, 



* In stating (/. c.) that the genus Caridina was not known to occur in West 

 Africa, I overlooked Hilgendorf's description (SB. Ges. naturf. Preunde Berlin, 

 1893, p. 156) of a species from Togoland. Bouvier has since recorded a variety ot 

 the same species from the interior of the French Congo and from the neighbourhood 

 of Lake Tchad. 



t It has lately been suggested by Dr. F. Sarasin (C. R. Congres Internat. Zool. 

 Berne, 1904 (1905) p. 151) that the peculiar richness in Decapod Crustacea which 

 distinguishes the fresh-waters of Celebes may be directly correlated with the poverty 

 of the fish-fauna of that island. It is plain that this explanation cannot be applied 

 to the case of Tanganyika, where the fish-fauna is remarkably rich. 



