404 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



pear to have been discovered to date in the Miocene 1 or in the 

 West American formations earlier than the Sooke Formation 

 of the Oligocene. As the number of described recent species 

 from the adjacent coasts is in excess of one hundred, the occur- 

 rence of most of which in the Pleistocene at least is undoubtedly 

 possible, it is evident that scarcely more than a beginning has 

 been made with the fossils. 



Material 



The material utilized in the preparation of this survey is 

 believed to include most of the extant specimens. It comprises 

 some 1065 valves, obtained chiefly from the following sources: 



1. The Delos and Ralph Arnold collections, the Arnold 

 and Hannibal Oligocene collection, and the Oldroyd collection 

 — all now contained in the museum of the department of ge- 

 ology, Leland Stanford Junior University. 



2. The museum of the California Academy of Sciences. 



3. The museum of the department of paleontology. Univer- 

 sity of California. 



4. The private collection of S. S. Berry. 



5. The private collection of Mr. and Mrs. E. P. Chace, 

 Los Angeles, California. 



6. The private collection of Dr. Frank C. Clark, Santa 

 Monica, California. 



7. The Mrs. Kate Stephens collection, now contained in 

 the museum of the San Diego Society of Natural History. 



Much of the more critical material studied from the private 

 collections mentioned has, through the generosity of their own- 

 ers, been assembled permanently in the author's study collec- 

 tion. 



The Chace collection has been of principal importance, as it 

 is not only extensive, but representative of several exposures, 

 some of them apparently owing their initial discovery and ex- 

 ploration to Mr. and Mrs. Chace, while the specimens as a 

 rule are accompanied by the most careful data. 



Dr. F. C. Clark's enormous material from the Santa Monica 

 Pleistocene is probably the most complete that has been taken 



1 cf. especially the check-lists of the Miocene Fauna in California Riven by J. P. 

 Smith in Proceedings California Academy of Sciences, (4), v. 3, p. 170-182, April, 1912. 

 I know of no additions since that time which would concern us here. 



