72 CALIFORNIA ACADEMY OF SCIENCES. 



species, as affected by local influences, have not yet 

 been obtained to explain these discrepancies. That these 

 were near the extreme altitudes at which thev could exist 

 in latitude 39°, is evident from facts obtained by Mr. 

 Raymond in latitude 38'. the relative position of the snow 

 line there being exactly in accord with the increased ele- 

 vation attained by the same species of Pisidium. Mr. Ray- 

 mond's collection also shows how much may be collected 

 on a hasty trip made for other purposes, and with little 

 spare time at most localities. Although many naturalists 

 have before visited Yosemite Valley none has reported 

 finding mollusca there, except Mr. F. A. Sampson, who 

 obtained }^itrince, and I never saw any species from the 

 upper Tuolumne Valley, except Physa blandi . 



It is true that the collection made in the Mariposa Big 

 Tree grove, about 5,500 feet elevation, contained three 

 or four land-shells not found on this trip, yet there may 

 have been errors as to their exact locality. The local in- 

 fluences of the groves are quite unlike those of the sur- 

 rounding regions, as shown by two or three other species 

 known to inhabit the Calaveras grove. 



The list of Mariposa grove species given in Bull. II, p. 

 359, shows that it is the upper limit of five helicoid spe- 

 cies, three of which are dwarfed by the climate, but the 

 subalpine species were found, on this trip, to extend 

 2,500 feet higher on the east slope without decrease of 

 size, while the aquatic species also retained full size. 

 Many of them being widely spread in the nearctic and 

 circumboreal provinces, are well known to belong to cli- 

 mates having as short summers and severe winters as the 

 subalpine zone of the Sierra Nevada. If any of them in- 

 habited this zone before the formation of the glaciers, 

 which once covered most of it, they must have been quite 

 .exterminated, and also down the mountain slopes far be- 



