1915] 



Clark: Fauna of the San Pablo Group 



479 



Named in honor of Dr. R. E. Dickerson, assistant curator of Palae- 

 ontology in the California Academy of Sciences, San Francisco. 



Dimensions. — Type specimen: length, 57 mm.; height, 37 mm. 



Occurrence. — A very common species in the Upper San Pablo 

 Group. Found associated in almost every locality with Cyrena cali- 

 f ornica Gabb, Macoma pabloensis, n. sp., Mytilus perrini, n. sp., Bit- 

 Mum? pabloensis, n. sp., Cerithium rodeoensis, n. sp. University of 

 California localities 16, 17, 1618, 1478, 1224, 1227, 1514, etc. 



MYA (CEYPTOMYA) OVALIS Conrad 



Plate 60, figures 3 and 4 



Cryptomya ovalis Conrad, Proc. Acad. Nat. Sci. Phil., vol. 8, pp. 312-316, 

 1856. 



Cryptomya ovalis Conrad, Pac. E. E. Bept., vol. 6, pt. 2, no. 2, pp. 69-73, 

 1857. 



The following is the original description of Cryptomya ovalis: 



Oval, compressed, posterior end truncated, umbonal slope angulated on the 

 umbo; beak medial; basal margin medially truncated; disk medially flattened. 

 Locality. — Monterey, Calif. Dr. Newberry. 



The species M. ovalis, as listed by the writer from the San Pablo 

 group, does not correspond very closely to Arnold's figure of Cryp- 

 tomya ovalis Conrad from the Etchegoin (Bull. U. S. G. S. 398, pi. 

 fig., also Bull. U. S. G. S. 396, pi. fig.). It would appear that Arnold's 

 M. ovalis does not differ very radically from BI. calif ornica, the Recent 

 species. M. ovalis of the San Pablo group corresponds very well to 

 Conrad's meagre description and the cut seen in the Pacific Railroad 

 Report, vol. 6. It differs from M. calif ornica in having more prominent 

 beaks ; the posterior end is truncated more nearly at right angles with 

 the ventral edge. The umbonal slopes stand out very prominently ; 

 the fold running down from the beaks to the ventral angle of the 

 posterior extremity is more prominent ; there is also a faint correspond- 

 ing ridge or fold running down from the beak to the anterior extremity 

 and the medium surface of the shell is flattened. 



Occurrence. — A very common species in the San Pablo Group. 



