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. 
