78 PALEONTOLOGY OF OHIO. 



dinal extremities. Area narrow, foramen wide and open, the cardinal 

 process being small and inclined inwards, does not fill the foramen, as is 

 the case in many species. 



Surface of the valves marked with from twenty-four to twenty-eight 

 sharp, angular plications, commencing near the beak, and continuing 

 simple for the distance of a quarter of an inch, beyond which point they 

 frequently bifurcate, or are increased by implantation until they become 

 fasciculate. This feature is only seen in the larger individuals, the young 

 or smaller specimens having the plications simple, or marked only by a 

 few added ones, the fasciculation not appearing until a more advanced 

 stage of growth. 



The shell somewhat resembles 0. plicatella, but differs in the extended 

 hinge line and in the inequality of the area of ihe two valves — that of 

 the ventral being much larger than in that species, giving an entirely 

 different form to the umbonal portion of the valve. It resembles much 

 more closely young individuals of 0. tricenaria, Conrad, but the plica- 

 tions on that species are always simple. Compared with 0. disparilis, 

 Conrad, from the Trenton limestones of Wisconsin, which appears to us 

 to be only a young specimen of 0. tricenaria, which species occurs abun- 

 dantly in the same vicinity as the original 0. disparilis came from ; it 

 differs in the greater convexity of the dorsal valve, and in the lower 

 cardinal area of the ventral, as well as in the duplication of the plica- 

 tions. 



Formation and locality : In the shales of the Hudson Eiver group in the vicinity of 

 Cincinnati, Ohio. We have seen specimens of what we believe to have been this 

 species in collections under the name of 0. disparilis. ^ 



LAMELLIBRANCHIATA. 



Genus PTERmEA, Goldf. 

 Pterinea demissa. 



Plate 2, fig. 1. 



Avicula demissa; Conr. Jour. Acad. Nat. Sci., Phil., Vol. VIII., p. 42, pi. 13, fig. 3. 

 Avieida demissa, Emmons; Geol. Eept. 2d Dist. N. Y., p. 404, fig. 2. 

 Avicula demissa, Hall; Pal. N. Y., Vol. I., p. 292, pi. 80, fig. 2. 



Shell sub-rhomboidal in outline, with the basal margin rounded; 

 hinge-line much longer than the body of the shell ; anterior wing ex- 



