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



men has since been lost and the figure is indeterminate, so its 

 real identity for the present must remain problematical. Very 

 possibly it represents an undescribed form. 



PLIOCENE 



Owing to the comparative scarcity of chiton remains in the 

 deposits of Pliocene age, they have been discovered at but few 

 localities, all southern and central California. In the litera- 

 ture the classification of the several formations in this part of 

 our area is rather confusing, so that which I have adopted is 

 in part tentative. 



Purisima Formation : 



3. Purisima, San Mateo County, California. "Crypto- 

 chiton c. f. stelleri" is reported from the "Neocene" at Purisima 

 by Ashley ('95, p. 327). As his work was done before the 

 local geological nomenclature was far developed, the exact 

 vertical position of his specimens is probably uncertain. 



San Diego Formation: 



4. Sea cliff at Pacific Beach, San Diego County, California 

 (Mrs. Kate Stephens). 



Fossils from this exposure have been listed and their rela- 

 tionships discussed by Arnold ( :03, p. 57-58, 60-64; :06, p. 

 28), who, however, records no chitons. The following are 

 now listed : 



Cryptochiton stelleri (Middendorff) 

 Ischnochiton conspiemts Carpenter 



Regarding one of these, Mrs. Stephens writes, "I got it 

 from the Pliocene beds, but I believe that Pleistocene beds are 

 found there, too, so it may have fallen down from that. I can't 

 be sure." It is apparent that there is here an uncertainty, prob- 

 ably applicable to all the specimens seen from this exposure, 

 which can only be removed by further work in the field. 



5. Rustic Canyon, Santa Monica, California (F. C. Clark). 



Mopalia sp. 



This locality is quoted as Pliocene on the authority of Dr. 

 F. C. Clark. I have not visited it, nor seen the association of 



