310 PALEONTOLOGY OF OHIO. 



V 



cardinal margin at an obtuse angle, and rounds abruptly into the base ; 

 anterior side wider (higher) than the- other, and more or less abruptly 

 rounded; dorsal margin depressed below the horizon of the beaks behind 

 the latter, where it is concave or nearly straight in outline, and inflected 

 so as to form a short corselet near the beaks, while in front of them it 

 slopes forward rather abruptly, and is provided with a well defined oval 

 lunule; beaks moderately prominent, rather gibbous, and incurved with- 

 out any obliquity or fissure, placed a little less than one-third the length 

 of the valves from the anterior margin ; posterior umbonal slopes form- 

 ing obscure subangular ridges which extend toward the posterior basal 

 extremity, but become obsolete before reaching it, while above this ridge 

 the posterior dorsal region is flattened, or a little concave, and smooth. 

 Surface ornamented with more or less defined concentric wrinkles and 

 lines of growth, which are crossed on the posterior-central regions by 

 linear, but distinct, raised, radiating costse, separated by wider depres- 

 sions. Of these costse the anterior sometimes descend almost vertically 

 from the beaks, with more or less curve, to the base, while farther back 

 they gradually become more oblique, and near the middle of the flanks 

 more closely arranged, but above and behind this they are more widely 

 separated again, and rrearly as oblique as the obscure umbonal ridge, 

 above which they are not defined. 



Length, 2.28 inches ; height, 1 inch ; convexity, about 0.85 inch. 



This shell strongly reminds me, by its general outline and physiog- 

 nomy, of those Jurassic species for which Prof. Agassiz proposed the 

 genus Cercomya. In that group, however, there is no lunule, and I am 

 not aware that any of the species of the same are marked by radiating 

 costffi as in the species under consideration. From all that is known of 

 its characters I am inclined to believe it more nearly allied to the curi- 

 ous Lyonsia-like carboniferous shells upon which Prof. McCoy originally 

 proposed to found the genus Sedgwickia, but which he afterwards referred 

 to the genus Leptodomus. Still it differs from this group {Sedgwickia) also 

 in the possession of radiating costae. These costse are not mere rows of 

 granules such as doubtless existed on nearly all the different types of 

 this family (Anatinidse), but decided costse, such as we see in Pholadomya; 

 and, what is rather singular, they do not exist on the anterior part of 

 the valves, but extend only as far forward as the beaks, under which 

 they end abruptly, the anterior one being as strongly defined as any of 

 the others, while only the concentric striae and wrinkles exist on the 

 anterior third of the valves, as well as on the posterior dorsal region. In 

 the possession of the radiating cost£e mentioned, as well as in the short- 

 ness of its hinge, the inflection of its cardinal margin, and in its general 



