244 GEIJLOGICAL HISTORY OF LAKE LAHONTAN. 



water mollnsks; but since a similar change would result from the necessary 

 chemical condition of the waters of an inclosed lake in which concentration 

 had been long continued, no definite conclusion as to the eflfect of the low 

 temperature seems possible. On the other hand, a warm Quaternary cli- 

 mate would presumably be favorable to the growth of mollusks, but even 

 if the climatic conditions were favorable, a more potent element in their 

 environment caused the shells to become depauperate. 



Mr. Call's studies have shown that the molluscan fauna of Lake La- 

 hontan was characterized by the predominance of the Limnceidce. This 

 family of mollusks at the present time is of world-wide distribution, but is 

 found most abundantly in cold-temperate and subarctic regions. During 

 the Quaternary it may be presumed to have had a similar isothermal distri- 

 bution. Thus in a very general way it might be inferred that the Quater- 

 nary climate in the region of Lake Lahontan was colder than at present. 

 The wide distribution of the Limnceidce, however, and their known powers 

 of enduring marked changes of environment, render this conclusion of 

 doubtful value. The majority of the molluscan species that inhabited Lake 

 Lahontan are still living in the Great Basin, and so far as this branch of 

 the palseontological evidence bears witness, we see no reason for conclud- 

 ing that the former climatic conditions differed materially from the present. 

 The only safe inference seems to be that the climate of the Great Basin 

 durino' the life of the mollusks we are considering' was not characterized in 

 mean temperature by extremes of either heat or cold. 



As regards the scarcity of food in Lake Lahontan, in reference to the 

 depauperation of its molluscan fauna, we know that the mollusks now found 

 fossil, like their living representatives, must have subsisted mainly on con- 

 vervoid growths. This form of vegetable life flourishes not only in fresh, 

 but also in brackish and alkaline waters, as may be seen in the various 

 lakes of the Great Basin at the present time. There is therefore no reason 

 to conclude from the probable composition of the waters of Lake Lahontan 

 that food of the character required by mollusks was not abundant. The 

 profusion of fos il shells in the sediments and tufas leads to the same con- 

 clusion, for without sufficient food molluscan life could not have been so 

 prolific. 



