20 Orthis (Flatystroplra) Cijplm—Jcvme?. 



tiseda, James, but the ventral valve of that species is not compressed, 

 is decidedly convex from the free margins, and the dorsal valve 

 concave, and it is a more robust shell than this, the radiating strite 

 finer, and internal markings quite different. 



Position and locality : Cincinnati Group, about 75 feet above low 

 ■\vater-mark of the river Ohio, at Cincinnati. 



Orthis (Platydwphia) cypJia. — (James.) 



ICypha, in allusion to the remarkable convexity of the dorsal valve,] 



Shell medium size; extremely gibbous; hinge line forming spine- 

 like projections, being over two thirds longer than the greatest breadth 

 of the shell below ; shell broader than long ; convexity equaling the 

 body below the hinge extensions ; cardinal area narrow, and finely 

 striated at rignt angles with the length. Dorsal valve, remarkably 

 fl^-'^ated mesial fold, with slopes commencing a little below the beak, 

 and extending to the front, at an angle of about 80 degrees to the 

 main body of the shell, where tliey turn at nearly right angles and 

 continue to the free margins ; beak incurved, not elevated above the 

 other valve ; the mesial fold consists of two costre. Ventral valve, 

 beak incurved, not elevated, nearly in contact with the other valve. 

 Sinus very profound, extending to the front, which is bent over to 

 nearly half the thickness of the shell beyond the cardinal line of the 

 dorsal valve ; one strong elevated plication in the center of the sinus, 

 and an obscure rudimentary one on each side ; lateral slopes concave. 



Twenty-two to twenty-six angular costee on each valve, about eight 

 of which commence on the cardinal line, and do not extend to the 

 beak. 



The line of junction of the two valves is nearly flat, or slightly 

 rouijded, and has a remarkably zig-zag appearance, forming, where the 

 sinus and mesial fold join, the letter W. 



Interior unknovin. 



"Width along the caixlinal line one and a half inches, half way 

 below the hinge line less than one inch ; length three quarters of an 

 inch. 



This shell resembles somewhat some of the larger specimens of 0. 

 crassa, Meek, but it is more gibbous, has a more profound and lengthy 

 sinus, greater length of hinge line, and finer and more numerous 

 costse. 



Position and locality : Cincinnati Group, "Warren county, Ohio. 



OvtJiis (Platydrophia) craxm. — (jMeek.) 

 In the Ohio Geological Survey, part ii., vol. i., Paleontology, page 



