554 CRETACEOUS PALEONTOLOGY. 



Antero-cardinal margin straight or slightly concave, sloping 

 downward from the beak; anterior margin rounding into the 

 basal margin, moderately convex throughout to the postero-basal 

 extremity, which is obtusely subangular ; posterior margin short, 

 truncated nearly vertically or slightly inclined; postero-cardinal 

 margin gently convex, sloping downward from the beak and 

 meeting the posterior margin in an obtuse angle. Surface of the 

 shell with an obtusely angular umbonal ridge, which passes from 

 the beak to the postero-basal angle in nearly a straight line, the 

 post-cardinal slope slightly concave to the cardinal margin ; the 

 post-cardinal margin sharply inflected to form a rather deeply 

 excavated escutcheon ; antero-cardinal margin inflected to form a 

 deep but rather ill-defined lunule. Surface of the shell marked 

 by regular, somewhat imbricating, concentric lines of growth, 

 and often by a, few broader concentric undulations towards the 

 margin. Hinge of the right valve with a strong cardinal tooth 

 transversely striate on its anterior surface, directly beneath the 

 beak. Behind it is a very large and broad triangular pit, with a 

 much smaller secondary pit just behind the lower end of the 

 tooth; in front of the cardinal tooth is a small triangular pit 

 about equal in size to the secondary pit behind, and in front of 

 this pit a low, obscure, tooth-like ridge extends obliquely forward 

 to the upper margin of the anterior muscular scar. M'uscular 

 impressions strong and about equal in size. Inner margin of the 

 free edge of the shell crenate. 



Remarks. The above description is based largely upon a very 

 perfect right valve from the Marshalltown clay-marl near Swedes- 

 boro. The more usual method of preservation in the Navesink 

 marl is in the form of internal casts. These casts have much the 

 same general form as the shell described, but are proportionately 

 lower, and the short, truncate posterior margin of the shell itself 

 is not differentiated from, the curving postero-cardinal margin; 

 the beaks are rather large, directed forward, and elevated above 

 the hinge-line; the muscular and pallial impressions are strongly 

 defined. These common casts of the Navesink marl were ap- 

 parently identified as C. vadosa, by Whitfield, at least in part, but 

 after a study of the types of that species as well as numerous other 

 examples from the South, it has not seemed possible to identify 



