180 PALAEONTOLOGY OF CALIFOENIA. 



truncated; base regularly convex. Surface marked by minute, 

 concentric strise of growth ; an angulated ridge runs from the 

 umbones to the posterior basal angle. Pallial sinus deep and 

 narrow. 



Length, 1.1 inch ; width, .8 inch; depth of one valve, .15 inch 



A single specimen, from the Martinez Group, Martinez. 



I have had this specimen a number of years, and have heretofore hesitated to 

 describe it, hoping to obtain others, which would settle its exact generic relations. 

 That it belongs to the Linnaean genus Mactra, and even to that genus as restricted 

 by Lamarck, I have little doubt; but from the nature of the matrix, and the 

 extreme tenuity of the shell, it is impossible to expose the hinge, and I am therefore 

 unable to assign it at present, with any degree of certainty, to its proper place 

 among the numerous closely allied genera of the family. 



CYMBOPHORA, K Gen. 



PL 29, Pig. 69. 



I propose this genus for one of the most common fossils of the 

 Californian Cretaceous, Mactra Ashburnerii. The hinge is com- 

 posed of a rather heavj 7 hinge-plate, bearing a cartilage-pit, not 

 sunk into its substance, as in the others of the Mactridce, but, as 

 it were, built up on its surface; a small, delicate, spoon-shaped 

 process, laid obliquely under the beaks, its base being on, or 

 slightly above the level of the hinge-plate ; in the right valve the 

 cardinal tooth is single, very delicate, and nearly at a right angle 

 with the anterior wall of the cartilage-pit ; in the left valve the 

 tooth is V-shaped, entirely separated from the pit, very slender, 

 and articulates between the tooth and the pit of the opposite side; 

 the lateral teeth are large and comparatively very robust. 



This genus seems to be most nearly allied to Trigonella, H. and A. Ad., as figured 

 in Genera of Recent Mollusca, but differs in the character of the cartilage-pit, in 

 the laterals being more robust, and in the cardinal teeth of the left valve being 

 more removed from the pit. 



