OEETACEOUS FOSSILS. 397 



inconspicuous beaks, which project but httle, and often not at all, beyond the 

 line of the hinge, and are situated near the anterior end, but not terminal ; 

 height and length of the valves subequal or longer than high ; anterior end 

 very short and rounded below the beaks, uniting in a more or loss regular 

 curve, according to the form of the shell, with the basal margin ; base gener- 

 ally somewhat regularly rounded, but often a little prolonged obliquely in 

 the direction of the umbonal ridge ; posterior end rounded, and the upper 

 margin gracefully curving forward to the extremity of the cardinal line ; 

 hinge line straight, usually half as long or more than half as long as the shell 

 below. Surface of the valves most convex on the umbones and along the 

 umbonal ridge, which is low, rounded, and not distinctly defined ; anterior 

 slope usuall}' more or less abrupt, and posterior slope and cardinal portion 

 of the valve flattened or but slightly convex. 



Surface of the shell marked by strong, rather closely arranged, sub- 

 angular concentric ridges, with regular concave interspaces. The lidges 

 are variable on different individuals, but on the same specimen usually 

 increase gradually and regularly from the beak outward with the increased 

 growth of the shell, and on some specimens which we have referred to the 

 species have finer concentric linings in the depressions. 



The specimens of this shell before us, to the number of more than a 

 dozen, varying from less than an inch to over four inches in height, are quite 

 variable in the degree of convexity and outline as well as in the propor- 

 tional strength and distance of the undulations, no two agreeing exactly 

 in any of these particulars. For this reason we have considered the forms 

 previously given as species and varieties cited above, which were obtained 

 from the same region of country as our specimens, as probably belonging 

 to one species. It is quite likely that those described by Dr. Roemer and 

 Professor Toumey, loc. cif., are also identical forms, and that they should be 

 all referred to I. proximus Toumey ; but not having access to specimens of 

 these we do not feel certain. Besides, those coming from distant parts of 

 the country are more likely to prove distinct. We notice among the speci- 

 mens of this type, as in those of I. sagensis, a tendency often to abrupt or 

 sudden geniculation of the valves after attaining a certain size, and in such 

 cases the concentric undulations are usually very faint or obsolete beyond 



