642 CRETACEOUS PALEONTOLOGY. 



shell. Hinge-line arcuate ; antero-cardinal margin sloping rather 

 abruptly forward to the anterior extremity of the shell below the 

 middle; basal margin convex anteriorly through the greater por- 

 tion of its length, becoming concave behind; postero-basal ex- 

 tremity angular ; posterior margin short, vertically truncate, curv- 

 ing rather abruptly above into the long sloping postero-cardinal 

 margin. Right valve strongly ventricose, with an angular um- 

 bonal ridge which is faint or obsolete towards the beak, becom- 

 ing conspicuous as it approaches the postero-basal angle oi the 

 shell ; in front of the umbonal ridge in the lower half of the shell 

 is a rather narrow but distinct sinus which forms the posterior 

 sinuosity in the basal margin; the post-umbonal slope concave. 

 Surface of the valve marked with nine or ten strong, rounded, 

 elevated, concentric costae, which continue from the anterior 

 margin of the shell to> the sinus in front of the umbonal ridge, 

 the interspaces about equaling the ribs in width. On the umbo the 

 concentric markings are reduced rather abruptly from the strong 

 costae tO' fine concentric lines; passing over the umbonal ridge 

 and down the posterior slope, are rather fine, sublamellose, con- 

 centric lines of growth. Left valve much less ventricose than the 

 right and the beak much less produced, the surface marked only 

 with more or less irregular concentric lines without the strong 

 costae. 



Remarks. — This species has a long range in the Cretaceous 

 beds of New Jersey, and can always be easily recognized by the 

 peculiar markings of the right valve which can frequently be 

 seen as impressions in the matrix. The species is quite charac- 

 teristic of the Woodbury clay, in which formation it sometimes 

 occurs in great numbers. In most of the other formations, how- 

 ever, it is comparatively rare. The only other New Jersey 

 species with which it is in danger of being confused is C. cliff- 

 woodensis which is much larger and lacks the sinus in front of 

 the umbonal ridge. The species is very similar to C. murchisonia 

 Lea from the Claiborne Eocene. 



Pormation and locality. — Merchantville clay-marl, near Ma- 

 tawan (loi), near Jamesburg (139, 140, 141), Lenola (163); 

 Woodbury clay, Lorillard (i0'2), near Haddonfield (164, 165, 

 168, 183); Wenonah sand, near Marlboro (130), near Craw- 



