CRETACEOUS FOSSILS. 187 



terior ear short, angular, posterior ear broad, acuminate; ante 

 rior and posterior margins nearly parallel for a short distance 

 below the ears ; base forming an excentric curve, most produced 

 behind ; there is no distinct division between the body of the 

 shell and the ears. Surface polished, and exhibiting only faint, 

 concentric undulations, corresponding to lines of growth, and a 

 few microscopic, radiating lines, posteriorly. 



Figure, natural size of the largest specimen. 



Locality: Division B., near Martinez, and Division A., at the Ranch of San 

 Luis Gonzaga, Pacheco's Pass. 



But few valves of this species have been found. The outline is approximately 

 restored from undulations on a young specimen, the one figured being so decom 

 posed as to show no lines of growth or other markings. 



INOCERAMUS, Sow. 

 I. PIOCHII, n. s. 



PI. 25, Fig. 173, and 174. 



SHELL small, thin, inequivalve, high and narrow. Right valve, 

 with the beaks, large, prominent, incurved, and placed directly 

 along the anterior margin ; posterior margin, to the most promi 

 nent part of the base, forms a pretty regular, elliptic curve; 

 anterior margin sinuated. Left valve, beak small, not projecting 

 beyond the hinge-line, which is transverse and nearly straight ; 

 anterior margin nearly straight, except towards the lower end, 

 where it bends round to meet the base ; posterior margin slightly 

 convex. Surface marked by small, concentric ribs, which some 

 times take the form of moderately large undulations ; this is 

 most strongly marked in the large valve. 



Figures, natural size. 



Localities: Division A., north side of Mount Diablo; also Tuscan Springs. 



Named in honor of Mr. Pioche, of San Francisco, who kindly loaned me the 

 first good specimens of the species I met with. 



Fragments and single valves of several large and moderate sized species have 

 been found at various localities in the State; but, on account of the number of 



