646 GEOLOGY OF OHIO. 



Opisthoptera obliqua, n. sp. 



Plate 49, Figs. 6—8. 



Shell small, triangular, highest in the posterior third, and thickest 

 along the oblique anterior side; hinge line as long as the shell, posterior 

 margin vertical, gently convex, basal margin narrowly rounded ; anterior 

 side very abrupt, almost flat, forming an angle of about 55° with the 

 hinge. Beaks rather large, prominent, but little incurred and separated 

 by a considerable interval in casts of the interior. Ligamental area very 

 wide. Surface marked with small, apparently bifurcating costse, of 

 which those near the cardinal margin are if anything smaller than else- 

 where. On the whole they may be described as subequal. 



This species doubtless is closely related to O. extenuata, the two 

 species being very similar in their anterior halves. Still, it is highly im- 

 probable that they will ever be confounded, since the}' are so different 

 posteriorly, this part of the outline being vertical and slightly convex 

 in the present form and deeply sinuate centrally and prolonged above in 

 that one. The ligamental area also is much larger in O. obliqua. Com- 

 pared with O. alternata it is found that the valves though more convex 

 are not as uniformly rounded, the anterior side more oblique and much 

 more abrupt, and the costae of nearly equal size instead of alternately 

 large and small. None of the other species are near enough to require 

 comparisons. 



Associated with O. obliqua we find Byssonychia richmondensis , another 

 species having the anterior side flat. But as it attains a much larger 

 size, is relatively much higher and almost erect, and has a much shorter 

 hinge, they are not likely to be confused. 



Formation and locality : Upper beds of the Cincinnati group, Rich- 

 mond, Indiana. It is there associated with O. casei and Ortonella hainesi, 

 Miller, sp. The type specimens were collected by Mr. John Misenfr, 

 and are now in the author's cabinet. 



Opisthoptera laticostata, n. sp. 



Plate 47, Fig. 6. 



Shell of medium size, rather long, obliquely triangular in outline, 

 widest in the posterior half, and most convex in the umbonal and anter- 

 ior parts; greatest height a little less than two thirds of the length. 

 Hinge line straight, extending the full length of the shell; posterior 

 margin somewhat oblique, on the whole sloping forward and forming an 

 angle of about 68° with the hinge; sinnate in the middle and gently 

 convex above and below; basal margin rounded in the posterior 

 half in front of which the outline ascends with a very gentle curve to 

 the lower extremity of the narrow byssal opening, when it turns abruptly 

 iipward and finally forward again to the beaks. The latter are large 



