CRETACEOUS FOSSILS. 183 



from T. Mathew&onii by its more symmetrical form, the outline forming a nearly 

 regular ellipse ; and from T. Hornii by that species being very much longer in 

 proportion to its width. 



T. UNDULIFERA, D. 8. 

 PI. 30, Pig. 74. 



SHELL moderately large, thin, slightly convex, very inequilat 

 eral, broadly rounded in advance, tapering behind; beaks two- 

 fifths of the length from the anterior end, slightly pointed in 

 advance; cardinal margin sloping rapidly and with a slight con 

 vexity to the posterior end, which is narrowly rounded; base 

 prominent, convex. Surface marked by concentric ribs, forming 

 large undulations towards the beaks, and gradually diminishing 

 towards the base. 



Length, 1.65 inch ; width, 1.4 inch ; height of single valve, .2 inch. 



From a soft, gray sandstone, west of Martinez, probably belonging near the 

 upper portion of the Martinez Group. 



The shell is, in all cases, so softened by decomposition that I am unable, in any 

 of the specimens, to expose the hinge, and therefore refer the species to the above 

 genus with some hesitation. Judging from the imperfect impressions left on some 

 internal casts, the hinge has been very delicate, and seems more like that of a 

 Tellina than of any other genus, though the external form and general appearance 

 render this determination somewhat doubtful. 



In outline the shell is not unlike T. ooides, but the posterior end is narrower, 

 the beaks are more prominent, the surface is more convex, and the style of orna 

 ment will serve at once to distinguish the two species. 



DONAX, Linn. 



D. LATUS, n. s. 



PI. 30, Fig. 76. 



SHELL broad, triangular, thin, very inequilateral ; beaks high, 

 placed about two-fifths of the length from the anterior end; sides 

 sloping, nearly straight, a rounded angle running from the beaks 

 to the posterior basal margin, leaving a narrow area parallel with 



