CRETACEOUS FOSSILS. 185 



In advance of this the surface is gently concave ; behind, it is 

 nearly flat. Surface marked by numerous, fine, dichotomous ribs, 

 except on a small space under the beaks, where they sometimes 

 become obsolete. Internal edge minutely crenulated. 



Figure, natural size, from an unusually large specimen. Generally from an 

 inch to an inch and a quarter long. 



Localities: Division B., Martinez, and Marsh's; most common at the latter 

 locality. 



M. CYLINDRICA, II. S. 

 PI. 25, Fig. 167. 



Shell elongated, subcylindrical, sides nearly parallel; beaks 

 anterior, nearly terminal, broad, not very prominent; anterior 

 end broadly rounded ; base nearly straight, sloping upwards 

 slightly in advance ; posterior end obliquely truncated ; a broad, 

 rounded, umbonal ridge, passes down from the beaks, gradually 

 losing itself in the general convexity of the shell. Surface marked 

 by a few faint, concentric lines of growth, which sometimes 

 become slightly lamellar. 



Figure, natural size. 



Localities: Pence's Ranch, Butte County; Tuscan Springs; also near Martinez 

 (Division A.). 



LITHOPHAGUS, Miihlf. 



L. OVIFORMIS, n. s. 

 PI. 25, Fig. 168. 



Shell small, ovoid, very gibbous ; beaks large, anterior, over- 

 hanging; cardinal margin slightly arched; anterior and poste- 

 rior ends round, basal slightly excavated, nearly straight. Sur- 

 face unknown. 



Figure, magnified to about three times the natural size. 



Locality: Found embedded in the oyster, fig. 101 ; from Cow Creek, Shasta 

 County (Division A.). 



This species differs from L. Riplnjanus (nob.) in its smaller size, more robust 

 form, more prominent beaks, and in being more compressed laterally behind. 

 pal. vol. I. — 24 



