76 PALEONTOLOGY OF OHIO. 



shell below. Ventral valve depressed, convex, most ventricose on the 

 umbo; area narrow, longitudinally striated, extending a little more 

 than two-thirds the width of the shell ; divided in the center by a 

 broadly triangular foramen, which is partly filled by the cardinal pro- 

 cess of the opposite valve ; beak very small. Dorsal valve flat in its 

 normal condition, though generally concave from compression ; area very 

 narrow. Interior of the dorsal valve characterized by a strong mesial 

 septum and highly elevated cardinal process, which is thin and not 

 bifurcate; also by strong projecting crural processes and distinctly lim- 

 ited muscular impressions of medium size. Ventral valve marked by 

 large cardinal muscular imprints, which are pointed anteriorly and 

 widely divergent, the adductor muscle imprints being small and situated 

 near the rostral portion of the valve ; teeth not strong ; musci>lar im- 

 pressions strongly limited. Near the margin of both valves in the 

 larger specimens there is a thickening of the valves, extending entirely 

 around the shell, except near the central portion of the hinge line. 



Surface of the valves marked by fine, closely arranged, radiating 

 strise, and by several strong concentric lines of growth, as well as by 

 very fine concentric lines, which are undulated as they cross the radi- 

 ating striae. 



The shell has much the form and appearance of 0. emacerata, Hall, 

 from the same geological formation, but is much larger and somewhat 

 thinner in proportion to its size than that species, and the internal fea- 

 tures are quite distinct. The ventral valve is not so distinctly carinate 

 along the median line, although sometimes slightly so. 



Formation and locality : In the shales of the Hudson Eiver group, near Cincinnati, 

 Ohio, but on the opposite side of the river. So far as we can ascertain, no specimens 

 ■ of the species have yet been found on the Ohio side. 



OeTHIS ELLA. 

 Plate 1, fig. 20. 



Orthis [?] ella, Hall ; 13th Kept. State Cab., p. 121, 1860. 



Orihis [?] ella, Hall ; 15th Kept. State Gab., p. 158, Exp. pi. 2, fig. 6-8. 



Orihis[?] ella, Hall; 24th Kept. State Cab., Exp. pi. 7, fig. 21. 



Shell small, ovate ; valves nearly equally convex ; hinge line extremely 

 : short, being scarcely more and sometimes less than one-third the width 

 ■ of the shell, and scarcely affecting the contour of the cardinal margin, 

 '■which slopes from the beak of the ventral to the lateral margins a little 



