Brachiopoda of the Cincinnati Group. 29 



of the same anterio-posterior dimensions. In these the lateral slopes 

 are very concave, and the anterio-lateral margins sinuous and strongly 

 converging toward the front. The mesial sinus is well defined, and 

 widens and deepens rather rapidly forward ; and as the mesial fold is 

 proportionally less elevated, the front is often thus caused to be dis- 

 tinctly sinuous in the middle. 



Old specimens become quite thickened within, and consequently 

 have the cavity for the muscular attachments in the ventral valve very 

 deep, and similar to the lynx. The surface granulations are usually 

 very beautifully preserved. 



Found at Madison, Richmond, Clarksville, and generally, wherever 

 the upper 200 feet of the Cincinnati Group is exposed. I have never 

 knowH it to be foimd at an elevation of less than 600 feet. 



Orthis borealis — (Billings, 1859). 



Shell rather under medium size, transversely oval-subquadrate, or 

 truncato sub-oval, the length and breath varying, with relation to 

 each other, from as 9 to 11 to 11 to 12 ; both valves convex; hinge line 

 shorter than the greatest breadth of the valves (which is usually a 

 little in advance of the middle), meeting the lateral margins at more 

 or less obtuse angles ; lateral margins rounding to the front, which is 

 rather broadly rounded, or possibly sometimes faintly sinuous at the 

 middle. 



Dorsal valve sometimes slightly more convex than the other, its 

 most prominent part being near, or a little behind the middle, usually 

 sloping rather more distinctly to the lateral margins than toward the 

 front, where there is generally a broad, very low, undefined promi- 

 nence, or mesial elevation ; beak moderately prominent and arched, 

 but not strongly incurved ; area about half the height of that of the 

 other valve, directed backward and more or less arched. Interior un- 

 known. 



Ventral valve most convex near the umbo, and sloping sometimes 

 rather abruptly toward the posterior lateral angles, while the anterior 

 central region is depressed, so as to form a broad, very shallow mesial 

 sinus, sometimes extending backward nearly or quite to the middle ; 

 beak more prominent than that of the other valve, rather abruptly 

 pointed, inclined backward and moderately arched ; area broad-tri- 

 angular, well defined, and tapering to the lateral extremities of the 

 hinge, inclined and a little arched backward with the beak ; foramen 

 rather narrow, so slightly higher than its breadth at the hinge. In- 

 terior unknown. 



