666 GEOLOGY OF OHIO. 



nearly straight for one-half of the length posterior to the beaks, declining 

 and very slightly convex anterior to them ; posterior end almost regularly 

 rounded ; ventral margin broadly yet distinctly convex, the curve accele- 

 rating slightly as we follow the outline to the upper part of the anterior 

 margin where it forms an obtusely angular junction with the sloping 

 antero-cardinal edge. Beaks small, appressed, situated one-fifth or a little 

 more of the entire length behind the anterior extremity. 



A faint posterior umbonal ridge and an undefined mesial depression 

 or mere flattening. Surface of cast with rather strong concentric striae 

 or furrows on the anterior half, very few of them passing over the um- 

 bonal ridge. On the posterior cardinal slope several obscure rays may 

 be observed. Anterior muscular scar large, acuminate-ovate, pointed 

 above, situated immediately in front of the umbones ; posterior scar 

 occupying the center of the cardinal slope, very large and elongate ; 

 pallia! line simple, rather distinct for so thin a shell. 



This is the type of the genus and the largest of the known species. 

 Compared with the other species it is distinguished by the great size and 

 the greater obliquity of the anterior margin. The back also is straighter 

 and the ventral margin more convex than in most of the others. 



Formation and Locality. — Upper beds of the Cincinnati group, 

 Waynesville, Ohio. 



Psiloconcha subovalis Ulrich. 



Plate 52. Figs. 5-7. 



Orthodesma subovale, Ulrich, 1879, Jour. Cin. Soc. Nat. Hist., Vol. ii, p. 82. 



In this species the shell is higher than in any of the others, the length 

 being usually but one millemeter greater than twice the height. Compared 

 with P. grandis, its nearest congener, it is further distinguished as fol- 

 lows: The post-cardinal margin slopes downward in a manner not to be 

 observed in that species, and this peculiarity causes an obliquity of the 

 posterior margin and a greater arcuation of the dorsum. The hinge line 

 is horizontal for a short distance in front of the beaks, and the whole 

 anterior margin is less oblique and more rounded. Finally, there seems 

 to be a difference in the surface markings, the concentric lines being of 

 nearly equal strength on all parts, which appears not to be the case in 

 that species. 



Formation and Locality. — The original types were collected at Mor- 

 row, Ohio, where they occurred in the middle beds of the Cincinnati 

 group. Recently specimens that cannot be distinguished were obtained 

 from the lower beds of the group by Mr. George Asherman and the 

 author. His came from Boldface creek, Cincinnati, while mine are from 

 a branch of Willow run, near Covington, Kentucky. 



