WEST COAST PULMONATA. 509 



supposed to have their eggs transported by adhesion to the 

 feet of birds, although No. 11 may, as before remarked, have 

 spread independently along the two ranges from the north; 

 3d, Nos. 24 and 25 (?), which may have been spread like 

 the Yitrinoid species; 4th, No. 40, probably in the same 

 way; 5th, Nos. 42 to 45, which may be spread by birds, or, 

 being semi-aquatic, by aid of floods. 



V. Considering that none of the Helicoid species are 

 found above 1000 feet east of, and 1400 feet west of the 

 bay, and that they could spread only by crawling (except 

 when shells or eggs were washed downivard for short dis- 

 tances without injury], we must conclude that they reached 

 the shores of the region by floods chiefly from the north, 

 and landed at heights between the present sea-level and the 

 elevations just given. As they can ascend with more diffi- 

 culty than Limacoids, they go less high up, and five hundred 

 feet ascent is a liberal allowance for them to have climbed 

 in any numbers. Subtracting this from their highest known 

 ranges, we may assume that they reached the east side of 

 the bay when the land was five hundred feet low^er than 

 now, the sea - shore being about two hundred and fifty feet 

 above the fossil bed of Walnut Creek, and as the land 

 rose, gradually spread downward into the valleys, and up- 

 ward on the hills. Those of the Santa Cruz range would 

 then have colonized that side when it was nine hundred 

 feet lower than now, which may have been about the same 

 period, as the more western range has probably risen more 

 in the same length of time than the eastern, and the whole 

 elevation has been during quaternary times. 



YI. The much less abundance and limited diffusion of 

 the species known from the counties north of the ba3^, within 

 the limits of the map, in spite of the moister and cooler 

 climate, can only be explained by the influence of vol- 

 canic forces there, and scarcity of lime along the 

 central ridge of the coast mountains. The twenty -one 



