140 PALEONTOLOGY OF OHIO. 



The surface of the shell in its original condition has been marked by 

 fine radiating striiB, which are still visible on the casts of some individ- 

 uals near the front of the valves, although the most of them appear to 

 be smooth, except for the concentric constrictions. 



This species is probably most nearly related to P. occidentalism Hall, 

 from the Onondaga salt group, Gait, Canada West ; it differs, however, 

 very materially from that one in the much greater depth of the dorsal 

 valve, and also in the relative thickness of the longitudinal septa — that 

 one having them very thick and strong, and in the finer striations of 

 the surface. From P. Littoni, Hall, another closely allied species, it dif- 

 fers in being more extravagantly gibbous and narrower, and in being 

 more finely striated. 



The species has been referred to P. ventricosus, Hall, but the dissimi- 

 larity is so great that a comparison of the figures of the two species on 

 the plate will not fail to strike the most casual observer. 



Formation aiid locality: In the Niagara group, at Greenfield, Darke county, Ohio. 

 Collected by Rev. Mr. Hertzer. 



LAMELLIBRAIS^CHIATA. 



Genus AMPHICGELIA, Hall. 



(Extr. 20th Kept. State Cab., 1865 ; 20th Rapt. State Cab., p. 339.) 



AmPHIC(ELIA (LePTODOMUS ?) COSTATA (ll. Sp.). 



Plate 7, fig. 23. 



Shell of medium size for the genus, broadly triangular ovate, with 

 prominent projecting beaks, which are strong, pointed, slightly incurved, 

 and nearly terminal. Valves ver}^ ventricose and inflated in the anterior 

 portion, gradually declining to the posterior margin, giving a broadly 

 cuneiform cardinal profile ; hinge line straight, about half as long as the 

 shell, abruptly bent nearly at right angles beneath the beaks. Anterior 

 end regularly and broadly rounded from below the hinge to the basal 

 margin, which is more broadly rounded toward the posterior portion ; 

 posterior extremity obtusely pointed at the place of the greatest length, 

 which is about midway between the hinge line and the base of the shell. 

 From the obtuse posterior point to the extremity of the hinge the mar- 

 gin is abruptly sloping, with a moderate degree of curvature. In a car- 

 dinal view the anterior end of the united valves appears to be almost 



