FOSSIL MOLLUSKS. 241 



luon in other localities at a corresponding geological hoi'izon. This is the 

 most abundant and characteristic fossil of the Lahontan sediments ; it occurs 

 from the base of the oldest of the tufa deposits all the way through the 

 series, and is one of three species of mollusks found living in Pyramid Lake 

 at the present day. 



All the fossil shells obtained are of recent species, and the majority 

 have been found living in the Great Basin. A comparison of the living and 

 fossil forms has shown that the fossils are de^^auperate, and exhibit greater 

 variations in the size, thickness and sculpture of their shells than occur in 

 living examples of the various genera and species when obtained from 

 regions where they find a congenial environment. In this connection Mr. 

 Call says: 



"The wide range of Pompholyx in Lahontan beds makes possible a 

 valuable comparison of the same species from localities representing stages of 



the lake widely separated in point of time Specimens taken from 



the lithoid tufa on Anaho Island, in Pyramid Lake, when compared with those 

 from horizons correlated with the dendritic period present the widest range 

 among individuals. The shells from both localities are higher than Pyra- 

 mid Lake form, are much thinner, and the coiling of the whorls is much 

 looser. The lithoid tufa specimens present a large proportion of costate 

 forms, the ratio being as 1 to 2, while in recent specimens the ratio is as 1 

 to 32. The recent species apj^roximate P. effusa, var. soUda Dall, while in 

 sculpture and elevation the earlier forms of the lithoid tufa approach nearest 

 to the typical P. effusa, Lea." 



Comparative measurements of Pompliolyx effusa from deposits of lower 

 and upper Lahontan beds, and of specimens found living in Pyramid Lake, 

 show that the average size of the fossils from the older horizon is below that 

 of the Pyramid Lake specimens ; while those from the upper Lahontan sedi- 

 ments are larger than the living examples. On comparing the size of the 

 Pyramid Lake specimens with the average of the same species from fresh 

 water localities in the same region, it was found that the shells from fresh 

 watei' were larger than those obtained from Pyramid Lake, which, it will be 

 remembered, is somewhat saline and alkaline (see analyses, ^jages 57 and 58). 

 A large number of measurements of fossil and living forms, shows that the 



MoN. XI 16 



