FRESHWATER CRABS 255 



The fresh-water crabs (Potamonidae) must have invaded 

 Central America from the south. The family is confined to 

 southern Asia, southern Europe, Africa, South and Central 

 America. Except for a few species in Mexico, fresh-water 

 crabs are entirely absent from North America, nor do we 

 possess any evidence of their ever having lived there.* A 

 comparison with the range of Unio is, therefore, of particular 

 interest. The two South American groups of fresh-water* 

 crabs apparently spread westward from eastern South 

 America, that is to say in a direction opposed to that taken 

 by the Unios. We need only consider the northern group 

 which, to judge from its range, is much the oldest. .'Dr. 

 Ortmann f distinguishes the three genera Kingsleya, Epilo- 

 bocera and Pseudothelphusa. Kingsleya only occurs in 

 Guiana, while Epilobocera is peculiar to the Greater 

 Antilles. The third genus, Pseudothelphusa, ranges from 

 the Amazon through Guiana, Venezuela and Colombia north- 

 ward as far as Mexico, and south-westward to Ecuador, Peru 

 and Bolivia. There are quite a number of endemic species of 

 fresh-water crabs in Central America. Yet are we to conclude 

 from this fact that a slow migration took place across the 

 long isthmus since Pliocene times ? On the contrary, if, as 

 Dr. Ortmann suggests, the genus Epilobocera arose in the 

 West Indies from some ancestral Central American Pseudo- 

 thelphusa, that event must have happened in much more 

 remote times. It is customary to assume that the great mass 

 of the South American fauna, including mammals, birds, 

 reptiles, fishes and invertebrates all surged across the newly 

 opened highway towards Mexico in the Pliocene Period. If 

 Epilobocera succeeded subsequently in crossing from Central 

 America on a land bridge to Cuba, Haiti and Portorico, how 

 can we account for the fact that the existing faunas of Central 

 America and the Greater Antilles do not show more affinity 

 to one another than they actually do ? As compared with 

 Central America the mammalian fauna of the West Indies 

 is strikingly distinct and poor in species. We have also to 

 take into consideration that certain species of Pseudothelphusa 



* Rathbun, Mary J., " Freshwater Crabs of America." 

 t Ortmann, A. E., " Distribution of Freshwater Decapods," pp. 306 

 309. 



