414 PROCEEDINGS OF THE ACADEMY OF [1894. 



It is very probable that farther investigations will prove that the 

 range of Caridina is a somewhat different one, since fresh-water 

 crustácea of smaller size are mostly neglected by collectors, and the 

 fauna of the fresh- waters of most tropical countries are very little 

 known. Accordingly, the view given above on the geographical 

 distribution of Caridina may, perhaps, have to be changed later. 



The distribution of the most extreme genus of the family, Atya, 

 is somewhat similar to that of Caridina. It is found, like the latter, 

 in the West Indies and Indo- Malaysia, but there are some modifica- 

 tions. One species is known from West Africa, which is identical 

 with another described from the Orinoco, and there is recorded one 

 species from the Cape Verde Island, identical with the common West 

 Indian form. The presence of identical fresh- water species, both 

 in the West Indies and in West Africa, is a very remarkable fact, 

 but not an isolated one among the Decapoda. We know another 

 group of fresh-water Crustacea which shows the same peculiarity. 

 Of the genus Palcemon there are three species described from West 

 Africa, two of which, Pal. jamaicensis {=vollenlxoven%) and Pal. 

 olfersi, are likewise present in the West Indies, and one, Pal. macro- 

 brachion, is closely allied to a West Indian species, Pal. acanthurus.™ 

 In Atya the identity of species of both continents bordering the 

 Atlantic is due, no doubt, to other reasons than in Paloemon, the 

 latter being a very recent genus, having immigrated to the fresh- 

 waters quite recently, while some species are now immigrating from 

 the sea to brackish and fresh- water. On the contrary, the immigra- 

 tion to fresh- water of the ancestors of Atya took place a long time 

 ago, and, I think, this fact indicates a former connection of Africa 

 and America. 



The other range of the genus Atya extends over the islands of the 

 Pacific from Sumatra to the Samoan islands. None is recorded from 

 southern Asia, from the islands of the Indian Ocean, or from East 

 Africa. 29 



The two species described by A. Milne- Edwards from New Cale- 

 donia, A. margaritacea and robusta, are very doubtful, as I have 

 stated above. I do not know another example of a fresh-water 



' 28 See Ortmann, ibid., p. 747. — Palczmon vollenhoveni is certainly the same 

 as Pal. jamaicensis; in the paper quoted I supposed them to be nearly allied, but 

 distinct species. 



29 OnlyHilgendorf (v. d. Decken's Eeisen, III, 1, 1869, p. 101) records a very 

 doubtful species from the Seychelles, belonging, perhaps, to Atyoida. 



