654 SYSTEMATIC PALEONTOLOGY 



Shell rather small for the genus and rather thin, compressed, subovate 

 to ovate-trigonal in outline ; umbones rising a little above the dorsal mar- 

 gin, their apices acute and prosogyrate, slightly anterior in position; 

 lunule and escutcheon clearly differentiated but very narrow because of 

 the compression of the valves; anterior end broadly and symmetrically 

 rounded in front of the umbones; posterior dorsal margin gently sloping; 

 lateral margin obscurely and obliquely truncate : base line rounding 

 smoothly into the anterior lateral margin, obtusely angulated at the union 

 with the posterior ; posterior keel obscure but persistent from the umbones 

 to the posterior ventral margin, better defined by the change in the direc- 

 tion of the growth lines than by any variation in the plane ; external sur- 

 face sculptured with a very irregular concentric lineation, sharpest and 

 most regular in the umbonal region, and occasional more or less accentu- 

 ated growth lines and resting stages; ligament external, lodged beneath 

 the umbones, the resilium buttressed ventrally by the posterior cardinal 

 which it has largely effaced; medial right cardinal stout, trigonal, sub- 

 umbonal, transversely striated laterally; anterior cardinal laminar; hinge 

 dentition in left valve restricted to two subequal cardinals, the posterior a 

 little the larger, both of them striated upon tbeir inner faces; no trace of 

 laterals developed but anterior margin of left valve and posterior margin 

 of right valve bevelled to function as laterals and received in shallow 

 sockets in the corresponding valve ; muscle impressions distinct, impressed 

 in the adults, placed high up at the distal extremities of the hinge ; pallial 

 line simple, distinct, rather distant from the base line. 



C. linteus Conrad has been considered, without justification, as the 

 young of some of the clearly allied and larger forms, such as C. vadosus and 

 C. subplanus. Aside from the fact that it shows no evidence of imma- 

 turity, the shell is thinner and more compressed and much less strongly 

 carinated posteriorly than C. vadosus of the same size. The resemblance 

 to C. subplanus is more striking, but the concentric sculpture is finer and 

 more sharply impressed in the former, and as a rule, the umbones are 

 set farther forward and are more strongly prosogyrate. 



C. linteus has a distribution in Maryland very similar to that of 

 vadosus. but is very much less prolific. 



