628 



PATAGONIAN EXPEDITIONS: ZOOLOGY. 



diversity, although favorably situated for migration. ^ Another group which 

 is widely distributed in the Archiplatan area, though not confined to it, is 

 the Amnicolid genus Littoridina (fig. 35). The absence of Amnicolidae 

 in the Amazon system is probably apparent rather than real, since prac- 

 tically no collecting of small or minute mollusks of any kind has been 

 done in that vast area. It is likely that Amnicolidae will be found there 



Fig. 36. 



Known distribution of Littoridina, a genus 

 of fresh-water snails. 



Known distribution of Chilinidae, a family 

 of fresh-water snails. 



in abundance. Diplodou, another genus which Dr. von Ihering considers 

 Archiplatan, is found almost all over the continent, and cannot fairly 

 bed aimed as of Archiplatan origin. It no doubt arose from the same 

 Brazilian (and ultimately Archhelenic) stock as Hyna, etc.; but like Unto 

 in the northern hemisphere, it is a hardy stock not highly evolved in its 

 phylum, hence probably old. Compare in this connection the Unionid 

 and Pleuroceratid fauna of Alabama with that of Georgia and the Caro- 

 linas. In these adjacent areas, which have been continuous land since 

 very early times, we have as much difference in the fresh-water faunas as 

 has been shown to exist between Archibrazil and Archiplata. Rich faunas 

 of fresh-water mussels and gastropods are rarely found in regions like 



^ At Davenport, Iowa, I have found LymncBa stagnalis on logs rafted down the river from 

 Minnesota, but the species has not been able to gain a place in this fauna there or farther south ; 

 nor have many of the other northern Lymnreida;, which must yearly be brought down on drift 

 wood during the spring floods. 



