284 . PALEONTOLOGY OF OHIO. 



region, in front of which it rises into a low, rounded, mesial fold, that is 

 rarely continued very obscurely visible to the umbonal region, with 

 sometimes the faintest possible tendency to become flattened on top ; 

 beak very short and incurved. Ventral valve having a more flattened 

 appearance, especially anteriorly, where it is impressed into a shallow 

 mesial sinus, which narrows rapidly backward, and is sometimes con- 

 tinued faintly to the umbonal region ; beak a little ventricose, but only 

 projecting slightly beyond that of the other valve, upon which it is 

 closely incurved. Surface of each valve provided with about eight or 

 ten strongly projecting, and sometimes slightly waved, concentric la- 

 mellse, which rise abruptly from the surface, excepting around the front 

 and lateral margins, where they are more crowded, and extend out hori- 

 zontally to considerable distances ; very fine concentric strise, and some- 

 times obscure traces of radiating markings, are also seen between and 

 upon the lamellae. 



Length of a rather transverse, mature specimen, exclusive of the ex- 

 tended lamellas, 1.05 inches; breadth of do., 1.59 inches; convexity, 0.60 

 inch. 



Internal casts of the ventral valve of this shell show that the impres- 

 sions of its divaricator muscles are, faintly marked, and, together, occupy 

 a comparatively large, somewhat fan-shaped space, extending forward 

 beyond the middle of the valre, while those of the adductor muscles ex- 

 tend about half the length of the divaricators between the upper ends of 

 the latter, and, as joined together, present a neat cordate outline, very 

 pointed below. Casts of the rostral cavity show it to be tapering, arched, 

 rounded, or a little flattened on top, and transversely striated, while on 

 each side of it the umbonal region shows a few granules, representing 

 minute pits in the interior of the valve. On casts of the interior of the 

 dorsal valve impressions of the quadruple muscles are usually well de- 

 fined, the upper pair being each rhombic, connected along the straight 

 inner edges, and sometimes unequally bilobed above, while the lower 

 pair are longer, and occupy a somewhat larger and obovate space, that 

 tapers below, and is a little obliquely truncated on each side above. The 

 cast of the rostral cavity is small, much arched, and rapidly tapering to 

 a point that looks as if it had filled a minute perforation in the point of 

 the beak. The internal mesial ridge is very small, or merely sharply 

 linear, and extends forward sometimes to the middle of the valve, and 

 backward to the point of the rostral cavity. The perforation of the ven- 

 tral beak seems to be small, judging from the cast of the rostral cavity. 



I have not had an opportunity to compare specimens of this shell with 



