ON LAND AND FRESH WATER SHELLS OF LOWER 

 CALIFORNIA. . 



BY J. G. COOPER, M. D. 



The long, mostly arid and barren peninsula lying chiefly 

 between the latitudes of 23^ and 32''' 30' resembles Florida 

 in barely reaching into the tropics, though, on account of 

 the mountainous character of its interior, only a very 

 narrow strip around the southern and eastern shores may 

 be supposed to be entirely free from frosts. 



It differs, however, from all the States of the Union, 

 and also of tropical x\merica in the comparatively scanty 

 rainfall which only approaches abundance to the south 

 of latitude 28° as summer rains, and from 28° to 33^ as 

 winter rains, most abundant on the highest and most 

 northern regions. Some of the mountains northward are 

 said to rise to over 10,000 feet elevation. Those of the 

 southern half do not rise above 8,000 as far as known. 



The influence of these climatic conditions on the mol- 

 lusca is shown by the fact that in Florida more than 50 

 species of terrestrial mollusca occur ; in the intermediate 

 State of Texas are over 40, while on the peninsula not 

 more than 24 have been discovered. It is probable, how- 

 ever, that several Californian species will yet be found 

 to extend along the mountain summits farther south than 

 yet known, only three species being so far discovered to 

 inhabit the regions on both sides of the boundary line. 



The remaining species are of more tropical groups 

 and mostly peculiar to the peninsula. Only two or three 

 occur also on the main land of Mexico, but what is most 

 remarkable, two species occur also in the similar arid re- 

 gions of western South America and nowhere in the in- 

 tervening moist tropical regions. Their supposed migra- 



2d Ser., Vol. IH. April 23, 1891. 



