416 PROCEEDINGS OF THE ACADEMY OF [1894. 



most important topographical barriers to distribution would be 

 widely extending oceans and large tracts of land without fresh- 

 water. The Pacific Ocean forms a barrier of the first kind, while 

 the second may be partly connected with the climatic conditions of 

 the warmer parts of the world. Smaller areas of sea and land, 

 however, ma}'' be crossed by some forms, as is shown especially in the 

 distribution of some species of Caridina and Atya so . The means of 

 distribution are certainly very limited, and therefore a great number 

 of species are confined to ver}^ narrow districts. 



Lastly, the ancient character of the family induces me to suppose 

 that there are also bionomic barriers, the Atyidce not being able to 

 immigrate to localities occupied by other fresh-water animals better 

 equipped for the struggle for existance. 



I regret very much that exact observations on the habits of the 

 species of Atyidce, on the biology and bionomy, are wholly absent. 

 It is very probable that the different genera and species on farther 

 examination will show some differences, especially that the best de- 

 veloped are more resistant to external influences. 



The conditions of geographical distribution of the Atyidce are as 

 follows : — 



1. The Atyidce cannot endure cooler climates. {Climatic barriers.) 



2. They are true fresh-water animals. (Oceans and tracts of land 



without water form topographic barriers. ) 



3. Being animals of an ancient type, they are probably restricted 



by the occurrence of other fresh-water animals. (Bionomic 

 barriers.) 



4. The faculties of distribution are very limited. 



The Atyidce are, therefore, confined to the fresh- waters of the 

 tropics and subtropics; the distribution of the genera and species, 

 especially of the more primitive ones, shows a remarkable character 

 of survival. Only Caridina and Atya are of a more recent charac- 

 ter, extending over continuous areas within the tropics. Because of 

 the antiquity of the family it has no relations among the recent forms 

 of the litoral regions of the tropical seas. 31 



30 Caridina typus, wycki, nilotica ; Atya scabra, moluccensis, spinipes. 



31 Such, relations to the Atlantic, Indo- Pacific, and Western- American 

 regions (see Ortrnann, Jenaische Denksckr., VIII, 1894, p. 76) are not at all 

 evident, none of the well-known genera or species heing limited hy the horders 

 of one of these regions. 



& í 



