Contributions to Invertebrate Palaeontology. 513 



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 beyond 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 interior of the dorsal 

 valve is marked by a moderately strong mesial septum, 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 dis- 

 tinct 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 hydraulic limestone of the 

 Lower Helderberg group, at Greenfield, Ohio. 



Genus PENTAJMERUS Sowerby. 



Pentamerus pes-oTis. 



Plate V, figs. 11-22. 



Pentamerus pes-ovis Whitf., Ann. N. Y. Acad. Sci., March, 1882, 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 narrow, and not strongly arched. Cast of the dorsal 

 valve characterized by a proportionally 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 valves has been destitute of plications, 

 but is usually marked in tlie larger individuals by several strong varices of 

 growth near the front margin, which give to the shell a prematurely old 

 appearance for so small a species ; the individuals seldom exceed five-eighths 

 of an inch in length on the ventral side. 



The species is unlike any known form of a similar size, in the 

 shallowness of the valves, in the erect character of the ventral beak, 



