WEST COAST PULMONATA. 357 



■sufficient data to follow it out, but until more is learned we 

 <}au only infer part of it from tlie present distribution and 

 variations of the living species. 



In 1869, I published what was then known on the sub- 

 ject, and again, in 1873, gave the special distribution of our 

 banded species and varieties with maps, in the Proc. Cal. 

 Acad., V, 121. Some additional information and cor- 

 rections have since then been accumulating, especially with 

 regard to the region around S m Francisco Bay, which is 

 the only well- explored region of land shells on the 

 west slope, and apparently the richest in variety of species, 

 subspecies etc., of any north of lat. 32'^ on this side. 



1. SIERRA NEVADA. 



Ill addition to what has been stated as to the occurrence 

 of the large Helicoids on the west slope of these mountains 

 only below the elevation of 5,000 feet, probably on account 

 of the absence of lima in a proper amount or condition 

 higher up, it must be noticed that the crystalline limestone 

 is not always sufficient to insure their existence even when 

 climate and moisture are favorable. As will appear later, 

 the lime must be a part of the soil, as the mollusca only ob- 

 tain it through the vegetation growing in that soil. But 

 above the limits of the large species there is found a grouj) 

 of small, often minute species, rare lower down, which shows 

 that sufficient lime exists in the vegetation above the lime- 

 stone strata to supply the little they need; and this doubt- 

 less comes from the less calcareous rocks, including the 

 volcanic. 



These small species include what are conveniently grouped 

 as Vitrinoid and Succinoid species, which are found 

 chiefly from 5,000 to 6,000 feet altitude in the mountains 

 near lat. 39'. Above that height I found no land shells; 

 though the bivalve Pisidium occidentale exists in the rail- 

 road pass at about 7,000 feet. From there up to 9,000 

 feet, where patches of snow lie permanently on Mt. Stan- 



