168 PALAEONTOLOGY OF CALIFORNIA. 



lines of growth. The cardinal margin is faintly truncated in a 

 similar manner to D. elevata, though in a less degree. 



Figure, natural size. 



Locality: On the north side of the Siskiyou Mountains, in Southern Oregon, 

 where it is not rare, though usually found as casts in the sandstone. 



This shell is about the size of, and resembles somewhat, D. tennis, Meek, from 

 Vancouver Island, but is less convex, broader, and not so high as that species. 

 It resembles it also in being very thin. 



D. GYRATA, n. 8. 

 PL 23, Fig. 148. 



SHELL lenticular, nearly circular; beaks small, central, in 

 clined anteriorly ; cardinal line curved, sloping downwards, and 

 uniting with the posterior margin ; length and breadth about 

 equal. Surface marked by numerous irregular lines of growth, 

 crossed by a few indistinct, radiating lines. Lunule small, deeply 

 impressed. 



Figure, natural size. 



Localities: Common at Marsh's, southeast of Mount Diablo ; also found at Mar- 

 tifiez and near San Diego ; and a single cast was found near the mouth of Canada 

 de las Uvas, at Alizos Creek (Division B.). 



The extremely compressed form of this shell is one of its most prominent cha 

 racters. The thickness of the specimen figured, measured from the external sur 

 faces of the shell, is not more than three-tenths of an inch. 



D. nSTFLATA, n. 8. 

 PI. 23, Fig. 149. 



SHELL subcircular, very convex; beaks small, prominent, in 

 clined strongly in advance ; height a little greater than the width ; 

 margins all regularly and nearly equally convex. Lunule none. 

 Hinge robust, internal margin plain. Surface marked by irre 

 gular, concentric lines of growth. 



Figure, natural size. 



Locality : Chico Creek, Butte County (Division A.). 



