40 PALEONTOLOGY OF NEW JEESEY. 



valve sometimes showing one and a half to nearly two volutions in well- 

 preserved adult specimens. Convex valve deep, and on the back strongly 

 angular, especially so in the earher parts formed, the portion near the beak 

 often being sharply carinate and smooth. Surface of the valve marked by 

 strong radiating costse, which are round on the surface, and separated by 

 narrow depressions. Costse frequently bifurcating, a;nd radiating from or 

 dividing along the umbonal ridge; one set curving toward the anterior side 

 and the other toward the basal margin. In old shells the costse are often 

 more or less obsolete beyond the middle of the shell, where the con- 

 centric lamellae, which everywhere cross the costse, become much more 

 developed and often form comparatively broad fringe-like expansions. Upper 

 or left valve flat or slightly convex, often becoming slightly concave toward 

 the antero-basal margin in advanced stages of growth, while in many in- 

 stances both valves conform in producing a deep sinuosity on the anterior 

 side below the beaks. Surface of the flat valve very strongly lamellose on 

 the posterior half, while showing incipient costse on the anterior side below 



the apex. 



The features of this shell, as it occurs in the Cretaceous beds of New 

 Jersey, are sufficiently well marked and distinctive to leave no room for 

 error in its identification, when medium sized or adult individuals are exam- 

 ined which preserve the normal form of the species. But sheik of all this 

 family are so extremely liable to distortion during growth, from external 

 influences, that individuals of any of them may occur which retain but few 

 of the normal features of their species, and this, like others of the group, is 

 occasionally met with departing widely from the general form of its kind. 

 They will be found to vary greatly in the strength and number of the costae, 

 and in their proportional strength, some examples having been noticed where 

 they were almost obsolete. The same example often shows them well de- 

 veloped on one part, and on another will show scarcely a trace of their ex- 

 istence. The angularity of the umbonal ridge of the right valve is also an 

 extremely variable feature, some having a broadly rounded form nearly to 

 the apex, while others will be extremely sharp and angular with a sub-spinose 

 carination. This difference is not unfrequently the result of the adhesipn of 

 the shell, by the anterior side of the beak, to foreign substances, though not 



