ye IV Species <if Fussils f rum (Jitiu. lU.") 



die of the valve, and six or seven may be couuted on each side of the vah'e. 

 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 wliieh is 

 smooth, and marked only hj concentric lines of growth. The interior of 

 the dorsal valve is marked by a moderately strong mesial septum, extend- 

 ing from the apex of the valve to about one-third of its length. The shell 

 appears to have been also marked by tine 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 sjjecies having close resemblance to it, 

 but none in rocks of corresponding age or position having very 

 close affinities to it. 



Formation and Locality. — In the hydraulic limestone of the 

 Lower Helderberg group, at Greenfield, Ohio. 



Pentaiiierus pes-ovi§, n. sp. 



Pal. O., Ill, Plate 1, Figs. 11—22. 



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

 pressed 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 pro- 

 portionally large, being long and narrow, and not strongly arched. Cast 

 of the dorsal valve characterized by a proportionally large and broad cardi- 

 nal 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 usuallj^ marked in the larger individuals by 

 several strong varices of growth near the front margin, which give to the 

 shell a prematurelj^ old appearance for so small a species ; the individuals 

 seldom exceeds 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, and in the deeply divided feature of the cast of this valve. 

 The dorsal valve is much less marked, and is often destitute of 

 any distinguishing feature. 



Formation and Locality. — In the hydraulic limestone of the 

 Lower Helderberg group, in Adams County, Oliio, occurring in 



