136 PALAEONTOLOGY OF OHIO. 



Rhynchonella TbNNESSEEjSTSIS. 

 Plate 7, figs. 16, 17. 



Shynclwnella Tennesseensis, Ecemer; Sil. Fauna West. Tenn., p. 72, pi. 6, fig. 14. 

 Rhynchonella Tennesseensis, Hall. 



Shell rather above the medium size, sub-circular or sub-triangular in 

 outline in a dorsal or ventral aspect, somewhat rounded at the beaks in 

 the cast, the condition in which it is mostly found in Ohio, and broadly 

 truncate in front, the cardinal margins being rounded to the widest part 

 of the shell, which is near the front of the valves. Dorsal valve very 

 highly convex, rapidly rising from the beak to the front of the shell, 

 along the center, but as rapidly sloping on the lateral portions from the 

 middle of its length to the antero-lateral angles; center of the valve 

 somewhat abruptly elevated forward of the middle, in some cases form- 

 ing the mesial elevation. Ventral valve shallow on the rostral and lat- 

 eral portions, very broadly and deeply sinuate in the middle, the depres- 

 sion occupying much the greater part of the breadth of the valve; front 

 strongly elevated, and prolonged in a broad linguiform extension, fitting 

 into and conforming to the elevation of the dorsal valve. 



Surface of the shell marked by from fourteen to twenty simple, strong, 

 angular plications, six to eight of which are elevated on the dorsal valve, 

 forming the mesial fold, and a corresponding number depressed on the 

 ventral valve to form the broad, shallow sinus. 



The specimens obtained from Ohio being in the condition of internal 

 casts, the rostral portion of the specimens is shortened by the removal of 

 the shell, which is much thickened in this part, and instead of the 

 pointed beaks of the perfect shell, shows the reverse of the hinge char- 

 acters, and the form of the muscular impressions. This latter feature is 

 large on the ventral side, giving an elongate ovate elevation on the pos- 

 tero-central region of the cast. On the dorsal side the impressions are 

 faintly marked, and are often distinguished' oii this valve only by the in- 

 terruption of the plications on this part of the surface; near the center 

 of the hinge are seen the impressions of the teeth and socket lamellae. 



The species is readily distinguished by the broad, sub-triangular form, 

 and coarsely plicated surface, together with the broad sinus and elevation, 

 especially as seen in a front view. 



Formation and locality : In the limestones of the Niagara group, at Yellow Springs, 

 Ohio. Collection of Prof. Hall, and Ohio State collection. Collected by Prof. Ed- 

 ward Orton. 



