414 GEOLOGY OF OHIO. 



particular. The species as recognized in the Silurian rocks of Perry 

 county, Tenn., resemble exactly this from Ohio, both in size and gen- 

 eral characters. It has proven hitherto quite rare, but might possibly be 

 found in greater abundance were it sought for; the specimens noticed oc- 

 curring on blocks of stone selected for other fossils. 



Formation and Locality. — Eower Helderberg group (Waterlime beds) , 

 at Greenfield, Ohio. 



Genus RHYNCHONEELA Fischer. 



Rhynchonella hydraulica. 



Plate I, fig. 17. 

 Rhynchonella hydraulica Whitf., Ann. N. Y. Acad. Sci., March, 1882, p. 194. 



Shell rather smaller than medium size, transversely oval in outline and ventri- 

 cose in profile ; the dorsal valve being highly convex, and the ventral somewhat de- 

 pressed convex. Beaks small, not prominent or conspicuous; that of the ventral 

 valve moderately incurved, and the other rather strongly incurved. Surface of the 

 shell marked by from sixteen to eighteen simple plications, four of which are 

 strongly elevated on the front half of the dorsal valve to form the mesial elevation, 

 which does not extend bej ? ond the middle of the valve, and six or seven may be 

 counted on each side of the valve. The plications are but slightly elevated, are 

 round on the summit, and do not extend beyond the middle of the shell, the upper 

 part of which is smooth, and marked only by concentric lines of growth. The in- 

 terior of the dorsal valve is marked by a moderately strong mesial, extending from 

 the apex of the valve to about one-third of its length. The shell appears to have 

 been also marked by fine concentric lines of growth, some of which form distinct 

 varices. 



This species belongs to the semi-plicated group of the genus, of 

 which there are many species having close resemblance to it, but none in 

 rocks of corresponding age or position having very close affinities. 



Formation and Locality. — In the h) r draulic limestone of the Lower 

 Helderberg group, at Greenfield, Ohio. 



Genus PENTAMERUS Sowerby. 



Pentamerui pes-ovis. 



Plate I, figs. 11-22. 

 Pentanierus pes-ovis Whitf, Ann. X Y. Acad. Sci., March, 18S2, p. 195. 



Shell quite small, and of a somewhat broadly triangular form, with depressed 

 convex valves, the ventral side being nearly twice as deep as the dorsal, and more 

 elongated at the beak, giving it the triangular character ; cardinal slopes straightened 

 and rapidly diverging; front broadly rounded. 



The species is known only in the condition of internal casts, and as thus seen, 

 the ventral valve is deeply cleft along the median line by the removal of the central 

 septum, the slit often extending more than three-fourths of the length of the valve. 

 The filling of the spoon-shaped cavity is proportionally large, being long and nar- 

 row, and not strongly arched. Cast of the dorsal valve characterized by a pro- 

 portionally large and broad cardinal plate, from which project two long and 

 strongly divergent and distant crural processes, reaching far along the surface of 

 the cast in some cases, while in others they are quite short. The surface of the 



