138 PALEONTOLOGY OF OHIO. 



This very variable species is so common and so well known that an ex- 

 tended description is unnecessary. It has been described under several 

 names, indicating varieties or phases, most of which can be found at 

 almost any locality where it occurs abundantly, and where the circum- 

 stances of life have been favorable to its development. The surface 

 characters are extremely variable, being smooth or variously plicated 

 over a portion only, or over the entire surface, the plications extending 

 only to a short distance from the front margin, or nearly or quite to the 

 rostral portion of the valve, sometimes finely marked, and, again, having 

 low, broad, and rounded ribs. The strength of the lobing or constrictions 

 of the valves may be either faint or strongly marked, and not uncom- 

 monly entirely obsolete. These several changes,, however, are only in- 

 dividual difi'erences, and of no specific importance. 



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

 Ohio. The species has a very wide geographical range, and is known in New York, 

 Ohio, Canada, Illinois, Wisconsin, and Iowa, as well as in many parts of Europe. 



t*ENTAMERUS (PeNTAMERELLA) VENTEIC6SUS. 



Plate 7, figs. 7, 8. 



Peniamerus ventricosus, Hall; Geol. Kept. Prog. Wis., 1860, p. 2. 



Pentamerus {Pentamerella) ventricosus, Hall; 20th Kept. State Cab., p. 374, pi. 13, figs. 

 18-21. 



Shell small globose, wider than long ; hinge line short, and abruptly 

 rounded at the sides. Ventral valve much the most convex, with 

 strongly incurved beak and prominent umbo, the middle of the valve 

 marked by a broad, deeply depressed sinus, extending from the beak to 

 the front margin of the valve ; front of the valve prolonged, and strongly 

 bent upward into a broad, linguiform extension. Dorsal valve promi- 

 nent on the umbo; beak small and incurved; center of the valve toward 

 the front sharply elevated, forming a narrow mesial elevation extending 

 about half way to the beak ; on the sides of the fold the surface is de- 

 pressed, forming a sinus on each side, and giving a strongly trilobed 

 character to the front of the shell. 



Surface of the shell marked by concentric undulations of growth, vis- 

 ible on the internal casts. Median septum of the ventral valve short, 

 not conspicuous. 



All the examples of this species seen, from localities in Ohio, are of 

 rather smaller size than those from the more western and typical locali- 



