300 CONRAD'S NEW FOSSIL SHELLS OF THE UNITED STATES. 



Miocene Species ? 

 OSTREA. 



Obtrea Titan. Elliptical or oblong; extremely thick and poaJerous, coufcractcd tovrards the hinge; 

 ligament cavity profound ; upper valve slightly convex ; surface coarsely laminated. Length 10 2 inches. 

 0. Titan, Con. Proceed. Acad. Nat. Sci., vi. p. 199. 



Locality. San Luis Obispo, California. 



This huge species is imbedded in friable limestone which contains abundance of 

 siliceous sand, rounded by attrition. No other fossil can be detected in the portions 

 of limestone which accompany the specimens. The cavity of the lower valve is 

 extremely capacious. 



OsTREA VESPERTiNA. Ovato-subfalcate, lower valve plicated or ribbed ; hinge long and wide, sharp and 

 somewhat pointed; ligament cavity wide, profound, minutely wrinkled; margins abrupt, cavity not 

 very deep; muscular impressions large, impressed; upper valve flat, irregular; pallial impression 

 crenulated throughout its whole extent, profoundly crenulatcd on the upper half of the shell. From 

 beak to base 11 ; transversely li. 



Locality. Near San Diego, California. Dr. Le Conte. 



This species has a remarkable resemblance, externally, to a variety of 0. horealis. 



The upper margin of the muscular impression is generally of a sigmoidal outline. 



