WEST COAST PULMONATA. 513 



bay, causing too rapid desiccation of the surface to allow of 

 the growth of seedlings, and favoring destructive fires. 



It now seems that the effects of the winds in the lower 

 parts of the Bay region, except so far as they spread fires 

 in the forests, are an advantage to pulmonates, which 

 have always been most numerous nearest to the 

 entrance of the bay. This does not, however, prove 

 that they are independent of forests, for these fur- 

 nish them with shelter and food more abundantly 

 than where no trees exist, so that the finest specimens 

 are found in the forests, though perhaps less frequent, 

 or harder to find. There is also a connection between cer- 

 tain forms and certain groups of trees, as I stated in the 

 synopsis, (Proc. Gal. Acad., III., 260 and 336-7, 1866). 



XIII. That the Bay region is, from physical configura- 

 tion, the best suited for commerce, and a large population, 

 of any on the coast, is a coincidence that may be favorable 

 to the increase of land pulmonates. We find already that 

 three species have become naturalized (though not desirable 

 additions) and the protection from fires, irrigation in sum- 

 mer, cultivation of trees, and destruction of many native 

 enemies, such as carnivorous quadrupeds and some birds, 

 may balance the injuries from cultivation. Some kinds are 

 indeed so numerous already as to be troublesome, especially 

 the Limacoids, in gardens near wet grounds. 



XIV. The great difference in distribution of species 

 near the bays from that in the Sierra Nevada may now be 

 explained. 



1st. The Sierra having been elevated probably before the 

 tertiary ejDOcli (though no terrestrial fossils yet prove ifc), 

 was a high range before the miocene land shells of Oregon 

 existed, and they extended over it at an elevation between 

 1000 and 5000 feet of its present height. 



2d. It has continued to rise during the tertiary epoch 

 and since, so that we find the large Helicoids dwarfed at 



35— Bull. Cal. Acad. Sci. II. 8. Issued August 25. 1887. 



