WEST COAST PULMONATA. 501 



The addition of Santa Clara Valley, makes a region more 

 similar in form and extent to that described east of the bay, 

 but there are 11 forms found there not known westward, 

 while only 3 occur westward, not found east. Probable 

 reasons for this will be given later, after adding species 

 found in the next county. 



It must be remarked that the ledges of limestone are not 

 so productive of land shells as the fossiliferous rocks, the 

 former being so siliciiied as to be usually little soluble. 

 One runs from Pt. Pedro southeast across the range at Black 

 Mt. to the east base of Mt. Bache; another forms a wide 

 belt around the south end of the spur west of San Lorenzo 

 Kiver. 



San Francisco County. 



Although only about six miles square and so long occu- 

 pied by a dense population, this county shows natural ad- 

 vantages for the land pulmonates, superior to any around 

 San Francisco Bay. These consist in its sub-insular posi- 

 tion causing a very uniform cool climate, moisture from 

 sea-fogs, and sufficient lime, supplied in part by the remains 

 of marine animals in lately raised beaches, in part from the 

 calcareous veins in the older sandstone. Even the drifting 

 sands that formed arid hills over nearly half its western 

 surface contain numerous fra^-ments of sea shells and micro- 

 scopic polyzoa, so that where vegetation could grow on them, 

 land shells of all kinds flourished, aided by the dense sum- 

 mer fogs. Yet the higher hills, chiefly metamorphic, al- 

 though having many rocks and trees to shelter them, show 

 the same absence of these animals as elsewhere, No. 30 and 

 varieties ascending only to about 400 ft. and No. 20 to about 

 900. I regret that I did not more carefully note the alti- 

 tudes to which Limacoids ascend in any of the counties, 

 but this could only be thoroughly done in winter, when the 

 higher regions are not easily explored. 



Although they were decimated by the domestic animals 



