84 PALEONTOLOGY OF OHIO. 



ally, however, examples are obtained preserving the surface markings 

 and other specific features. The form of the shell is elongate ovate, 

 widest within the posterior third of the length; hinge line distinctly 

 arcuate throughout, and strongly bent beneath the beak, the shell being 

 contracted in width at this point, giving a narrow, sharply rounded an- 

 terior end, which is longest below, the line of the middle of the shell. 

 Basal line sinuate a little anterior to the middle, corresponding to a 

 broad, shallow, and undefined sulcus which crosses the valves ; posterior 

 end strongly rounded ; shorter above the central line than below, and 

 gradually rounding into the posterior cardinal border. General surface 

 depressed convex; beaks small, compressed, and but slightly elevated 

 above the limits of the hinge plate. This latter feature is considerably 

 changed in many of the casts by the removal of the (substance of the 

 shell from between the beaks, leaving a much greater depth below the 

 beaks than is really the case with the shell itself, giving them the ap- 

 pearance of considerable elevation. 



Surface of the valves moderately convex ; most flattened toward the 

 anterior end; the middle of the valve marked by a broad, undefined 

 sulcus; the whole marked by fine, irregular striae, and a few stronger 

 lines indicating stages of growth. Anterior muscular scar large, rather 

 strongly marked ; posterior scar large, and very faintly marked. 



This species difi'ers from M. pholadiformis, Hall, in being destitute of 

 the diverging plications which characterize that species; and from 

 M. concentrica, herein described, by its greater size, and by the absence 

 of the regular, concentric plications on the posterior slope, as seen on 

 that species. 



The specimens occur usually much compressed, so that the valves 

 show but little convexity ; but some of those obtained have been com- 

 pressed vertically, producing an unnatural degree of convexity, and con- 

 siderable angularity along the umbonal ridge and on the central parts of 

 the shell — at the same time shortening the vertical height of the speci- 

 mens. 



c 



Formation and locality : In the shales of the Hudson River group, near Cincinnati, 

 and Waynesville, Ohio. 



