CRETACEOUS FOSSILS. 163 



V. TETRAHEDRA, 11. 8. 

 PL SO, Fig. 247. 



Shell small, thin, subquadrate; beaks small; anterior end 

 broadly rounded below, excavated under the beaks; cardinal 

 margin sloping posteriorly, nearly straight; posterior end ob- 

 liquely truncated; basal margin convex. Surface polished, 

 divided by a broad, rounded, umbonal ridge, into two planes, so 

 that when the two valves are in apposition they present four 

 well-marked curved faces. 



Figure, a little more than twice natural size. 



Locality: Not rare in the vicinity of Martinez (Div. A.). 



MERETRIX, Lam. 

 M. Uvasana, Con. 



PL 30, Fig. 248. 

 (M. Uvasana, Con. P. R. Pv. Report, vol. 5, p. 320, pi. 2, fig. 3.) 



Shell thick, oval, robust, a fourth longer than wide, very 

 inequilateral ; beaks large, strongly incurved, placed less than a 

 third of the length from the anterior end ; buccal margin pro- 

 minently rounded below, deeply excavated under the beaks; 

 cardinal margin sloping very convexly towards the posterior 

 end, which is subtruncatcd. Surface marked by small lamelli- 

 form ribs, separated by spaces about equal to four times the 

 thickness of the ribs themselves; the interspaces are sometimes 

 plain, sometimes striated. Inner margin plain. 



Figure, natural size. 



Localities: Abundant near Fort Tejon, whence it was described by Mr. Conrad ; 

 also found, not rarely, near Martinez, and nearly everywhere in Division B. 



This is the largest species of the genus, and one of the commonest fossils in 

 California. The figure given in the Pacific Eailroad Report is from a fragmen- 

 tary specimen, and conveys a very incorrect idea of the outline of the shell. 

 There can be no doubt of the identity of the present form with Mr. Conrad's 



