356 CALIFORNIA ACADEMY OF SCIENCES. 



The deposit in which they were found is considered "mio- 

 cene from the bones of extinct mammalia fonnd in it, and 

 the land shells help to confirm this. 



The extinct Mesodon seems to be the ancestor of the spe- 

 cies found in the regions north and westward, though not 

 very similar to any of them, while the close resemblance of 

 the other two western forms to living examples is remark- 

 able, for fossils so far anterior in time. The Patula is the 

 only one generically allied to the numerous forms of that 

 type now existing in the " Central Province," but unlike 

 them all. 



The isolated occurrence at present of the Gonostoma 

 about the caves of Calaveras County gives a clue to the ex- 

 planation of the similar isolation of several other west-slope 

 species, such as Polygyrella in Montana, and Polygyra 

 harfordiaaa in Mariposa County, Cal., which may also have 

 ranged widely during the tertiary epochs. 



Other geological evidences show that since the miocene 

 epoch the Cascade Eange and Sierra Nevada have been ele- 

 vated much higher than before, together with the *' Central 

 Proviacs" east of them, wJiile at the same time vast out- 

 flows of lava devastated the latter regions. These shells tend 

 to prove that at the time they lived in Central Oregon, that 

 region had a much]moister and milder climate, like that now 

 found west of the Cascades, and at a gradually rising eleva- 

 tion on the west slope of the Sierra Nevada, as we progress 

 southward. 



They also make it appear probable that any terrestrial 

 fossils found west of those ranges should be considered as 

 pliocene or later, although we have fresh-water bivalves in 

 the lignitic beds of Mt. Diablo, and it is possible that the 

 forests producing the lignite also contained eocene or older 

 land shells. 



B. — GEOaRAPHICAL DISTRIBUTION. 



It would be interesting to continue the subject of deriva- 

 tion from fossil species down to the present time, if we had 



