510 CALIFORNIA ACADEMY OF SCIENCES. 



forms known there, favored by climate, have extended them- 

 selves farther eastward than south of the "Golden Gate,' 

 and several large kinds are found east of the volcanic belt, 

 that probably reached there before the last period of vol- 

 canic activity buried the "fossil forest," and much life with 

 it. The occurrence of a few species throughout the volcanic 

 region, and of several others at points near its borders, 

 shows that Limacoids spread most rapidly over it, and those 

 with thin shells next, Avhile the large Helicoids were last to 

 obtain a foothold on it. 



YII. While it might be supposed that Nos. 30 to 35 

 could have more easily reached the Bay region from the 

 southward, on account of the courses of most of the present 

 streams, there is no fossil evidence that they ever existed 

 in that direction south of Monterey Bay; while Nos. 26 to 

 30, and 36, still have their living allies toward the north. 

 That they spread southward at different periods seems also 

 proved by the various distances they have reached, and by 

 the few fossils known. 



YIII. Thus we do not find that No. 36 was ever able to 

 cross the salt waters of the Golden Gate to San Francisco 

 County, but can easily believe that it could have been 

 washed down Suisun Creek and across the strait during the 

 winter floods, landing nearly opposite Mare Island, and 

 thence spreading along the moist western slope of the hills 

 to its present terminus, thirty-six miles southward. That it 

 is a very late colonist there, is also proved by its not having 

 been carried across Santa Clara Valley to the Santa Cruz 

 Mountains, which are better suited for it, while most of the 

 forms of 26 and 30 seem to have drifted over there, and 

 flourished more generally than on the east side. The five 

 or six Helicoid forms out of the tweniy-one found in San 

 Francisco County probably reached there in that way, the 

 others coming in the general modes before mentioned. Had 

 they been carried there by floods from the large rivers 



