SUBALPINE MOLLUSCA. 73 



low, judging from their present distance from the snow. 

 They have therefore extended their range upward 6,000 

 to 8,000 feet since the glaciers retreated, now occupying 

 the very channel cut by the ice into the solid granite, 

 between 4,500 and 9,700 feet elevation, for a distance of 

 over 30 miles east and west. Such an extension must 

 have been exceedingly slow, especially for the aquatic 

 species moving up against the streams, unless aided 

 by birds, which might have carried shells or their eggs 

 adhering to their feet. The glaciers in this latitude ex- 

 tended east to Mono Lake, giving them a total breadth of 

 over 50 miles, and the highest peaks now rise over 13,000 

 feet above the sea. 



We see here the same difference in elevation of several 

 species on opposite slopes as in latitude 38°, the land- 

 shells especially going higher on the eastern slope. This 

 difference may be caused by the greater amount of snow 

 falling on the west slope, which must lie longer on the 

 ground and shorten the active season for these animals, 

 though the average temperature must be milder on the 

 west slope and also moister, which may favor the exist- 

 ence of some of the larger species, not found eastward, 

 such as Cam^ylcBa mormonum and Triodopsis loricata, 

 up to Alta, 3,625 feet, and to Mariposa Big Trees, 5,500 

 feet. There is also a marked absence of limestone be- 

 tween 5,000 and 9,000 feet on the west slopes in both lat- 

 itudes. The little lakes on the west slope in latitude 39° 

 above 3,600 feet seem very destitute of mollusca as are 

 the violently running streams. 



For comparison I give a list of the species so far col- 

 lected in the subalpine region near latitude 39", showing 

 that although the number is about the same, the species 

 found differ in a marked degree, chiefly from local 

 causes. 



2d Ser., Vol. III. ( 7 ) August 8. 1890. 



