690 SYSTEMATIC PALEONTOLOGY 



lateral margin increasing the angle to 130 or 140 ; anterior end of shell 

 broadly and smoothly rounded, posterior obliquely and somewhat obscurely 

 truncate; base line asymmetrically arcuate, more strongly upcurved in 

 front than behind; a narrow posterior area rudely differentiated by the 

 increased prominence of the incremental sculpture, the broad and very 

 shallow depression which is often developed in front of it, and the still 

 more shallow depression along its medial portion ; external surface incre- 

 mentally sculptured, the striations sharp and regular in the immediate 

 vicinity of the umbones, almost obsolete over the medial portion and 

 irregular with occasional resting stages toward the ventral margin, uni- 

 formly coarse over the posterior area from the umbones to the base ; liga- 

 ment submarginal, opisthodetic, supported by a robust nymph; hinge 

 plate heavy, hinge armature restricted to three cardinals in each valve, 

 radiating fan-like from beneath the umbones; anterior cardinal of right 

 valve laminar, middle cardinal stout and trigonal, inclined forward, pos- 

 terior cardinal obliquely elongated, deeply sulcated medially; anterior 

 cardinal in left valve rather stout, expanded ventrally; middle cardinal 

 trigonal, inclined backward; posterior cardinal sharp and laminar, par- 

 tially fused with the dorsal margin; adductor muscle scars obscure, the 

 anterior elongated, the posterior semi-elliptical, placed high up under the 

 distal extremities of the dorsal margins ; pallial line simple but truncated 

 behind, rather distant from the base ; inner ventral margins simple. 



Dimensions. Altitude 76.5 mm., latitude 86 mm., semi-diameter 

 23.5 mm. 



Type Locality. Brightseat, Prince George's County. 



This species is much the largest and heaviest member of the genus 

 described from the East Coast or Gulf Cretaceous. C. alia, its probable 

 analogue in the Southern Atlantic Cretaceous, is smaller, relatively higher, 

 and more smoothly rounded, especially along the posterior lateral margin. 



The variation in outline is rather wide, the voung are much more 



/ o 



rounded, relatively lower, more evenly inflated and more symmetrical than 

 the adults, while the adults vary quite widely among themselves in relative 

 proportions and in the size of the umbonal angle. The figured specimens 

 are rather extreme but by no means unusual types. 



