392 CRETACEOUS PALEONTOLOGY. 



shell slightly angulated along the postero-umbonal slope and 

 very convex; marked by numerous strong concentric lines of 

 growth at irregular distances; no radiating striae. Hinge-plate 

 narrow in small and medium sized specimens and the teeth 

 small, but barely bent down at their inner extremity and few in 

 number ; the denticulations along the middle of the hinge vertical 

 and small. On large individuals the outer teeth are strong, from 

 four to five in number on each side, according to the size of the 

 individual; slightly declining outwardly, and the bent portion 

 usually nearly half as long as the horizontal portion, the bend- 

 ing being at an angle within ninety degrees, the denticles on the 

 middle part of the hinge being small and numerous. Muscular 

 scars, as seen on the casts, strongly marked; the impression of 

 the ridge deep, strongly arched, and situated pretty well up on 

 the posterior slope; surface of the cast marked by rather strong 

 vascular lines. The outer margin of the cast is bordered by a 

 strong keel, indicating the great thickening of the valves along 

 the pallial line, which extends around three sides, being broadest 

 on the anterior." (Whitfield.) 



The dimensions of a large left valve are: length 75 mm., 

 height 70 mm., convexity 26 mm. 



Remarks. This species may be easily recognized at all times 

 by reason of its subglobose form. Its usual mode of occurrence 

 in New Jersey is in the form of internal casts, and these differ 

 from similar specimens of C. tippana in the more rounded out- 

 line and the less obliquity of the shell, and in the absence of the 

 conspicuous subangular umbonal ridge with the more or less 

 abrupt posten>dorsal slope of the shell. The species also attains 

 a larger size than C. tippana. *The species occurs most abundantly 

 in the Merchantville and Navesink formations, it being the most 

 characteristic member of the genus in the first of these forma- 

 tions. The species has not been recognized in any beds higher 

 than the Navesink, its vertical range being quite different in this 

 respect from that of C. tippana, which is rare in the Merchant- 

 ville, and first occurs in abundance in the Marshalltown, and then 

 continues as a conspicuous, species up to the Tinton beds, except 

 in the Navesink. Although this species and C. neglecta both 



