WEST COAST PULMONATA. 511 



they would probably have lauded as ofteu ou the uorth 

 shore of the Goldeu Gate, aud become as pleuty iu the cool 

 damp localities ou the west side of Marin County, and more 

 common on tlie islands of the Bay. 



IX. The migration of No. 36 being thus explained, and 

 the general course of distribution of other Helicoids indi- 

 cated, Ave can now see how Xo. 37 may be derived from C. 

 traskii by a migration from the Sierra Nevada, but in an 

 opposite direction. The sketch of the distribution of C. 

 trcishii given on pps. 361 to 364 shows that it intergrades 

 wdth No. 38 on the coast southward, which may sufficiently 

 account for the origin and range northward of 38 and 39, as 

 they cannot be traced to any form now living within 500 

 miles north of No. 39. By a quicker route C. trasldi might 

 have easily been washed down the San Joaquin Valley to 

 the east slope of the coast mountains almost anywhere, but 

 did not find a suitable region for increase until reaching the 

 gap of the Bay region. Any of them landing on the north 

 shores of the strait would ascend along the banks of streams 

 and thus spread to their present northern limit about 50 miles 

 north of the Bay, but have not crossed the volcanic belt to 

 the west side of the Coast range. South of the straits we 

 also find that they have not gone west of the figure 485 in Ala- 

 meda County, though an allied form reaches Salinas Eiver 

 from the southward. This seems a more natural mode of 

 distribution for this form than that before suggested. 

 Here again the present location of its nearest allies 

 points to its origin, while on the other hand that of 

 the Ariontce is as plainly traceable to the northern 

 coast ranges instead of the -Sierra Nevada. The species 

 found there could apparently be as easily washed down, but 

 seems not to have become colonized. 



X. Although there is such a general resemblance in form 

 between Nos. 32 and 37 that Mr. Binney has considered 

 them closely related and mixed them in his figures, I con- 



