SEMI-FOSSIL SHELLS. 245 



The fossils that might be expected to throw the most Hght on the cU- 

 matic problem are the mammalian remains, but, unfortunately, up to the 

 present time these have beeti found in such limited numbers that but little 

 evidence as to the nature of former climatic changes can be derived from 

 them. 



Throughout Lahontan history Pompholyx effusa was the most abundant 

 species in the molluscan fauna, but only a very few individuals of this genus 

 have been found living in the present lakes of the basin. Moreover, the 

 dead shells of Pyrgula Nevadensis occur in profusion on the shores of Pyramid 

 and Walker lakes, but have not been discovered among Lahontan fossils. 

 This species is probably now living in the lakes of the region, as is indi- 

 cated by the fresh appearance of the shells, in some of which the soft parts 

 of the mollusks are still adhering. 



The occurrence of Pompholyx throughout the Lahontan series and its 

 rarity in the existing lakes of the basin, as well as the absence of Pyrgula 

 from Lahontan fossils, and its abundance in a semi-fossil condition, are be- 

 lieved to indicate that there was an interregnum between the time of Lake 

 Lahontan and the beginning of the present lakes of the basin. If our read- 

 ing of the records is correct, this time of change was a period of extreme 

 desiccation during which the lakes of the region evaporated to drj'ness, their 

 salts becoming buried beneath playa deposits, and their molluscan life nearly 

 if not completely exterminated. 



The absence of Pyrgula in Lahontan sediments, and its abundance in a 

 semi-fossil condition about the shores of the present lakes, in which it is 

 now rare, seems explicable on the assumption that it was introduced into 

 the basin at a recent date and found a congenial habitat in the mildly saline 

 waters of Pyramid and Walker lakes. Subsequently these lakes became 

 too saline and alkaline for its existence, and it was neai'ly if not completely 

 exterminated, so far as they are concerned. The present chemical compo- 

 sition of the lakes in question is believed to indicate about the limit of 

 salinity or alkalinity that fresh-water mollusks can sustain. 



The cases of a minute crustacean of the genus Cypris occur through- 

 out the Lahontan sei'ies, and at times are so abundant that they form the 

 principal portion of strata for several feet in thickness, as may be seen in 



