6 42 . GEOLOGY OF OHIO. 



Allonychia subrotunda, n. sp. 



Plate 48, Figs. 8-9. 



This rather small shell is of a more rounded form than A. jamesi, 

 the height being proportionally less and but little greater than the length. 

 Another difference in the outline consists in the greater prominence of 

 the central part of the anterior margin. Indeed, this part _ projects 

 beyond the byssal lobe. The radii number about fifty. 



Formation aud locality: Middle beds of the-Cincinnati group, Cin- 

 cinnati, Ohio; about 400 feet above low water mark in the Ohio river. 



i 



Genus OPISTHOPTERA, Meek. j 



Subgenus Megaptera, Meek and Worthen, 1866. Proc, Chicago Acad. Nat Sci., Vol. 



I, p. 22. (Not Megaptera, Gray, 1846.) 

 Opisthoptera, Meek, 1872, Proc. Acad. Nat. Sci., Phtla., p. 319; also 1873, Ohio Pal., 



Vol. I, p. 131 (note). 



Shell equivalve, usually triangular in outline, with the beaks of 

 moderate size, incurved and terminal, and the hinge line straight and 

 very long, in most cases forming a great posterior wing; length greater 

 than the height; anterior side more or less abrupt. In the typical sec- 

 tion the greatest height is in the anterior half, and the surface marked 

 with numerous and frequently bifurcating costse. In another group of species 

 provisionally regarded as congenerie, the posterior part of the shell is the 

 highest, and the radiating costse few and mostly simple. Byssal opening, 

 muscular scars and pallial line as in Anomalodonta and Byssonychia. 

 Hinge with two small cardinal teeth in each valve, but so far as known, 

 no posterior lateral tooth; external ligamental area usually narrow; no 

 internal ligament. 



Type: Ambonychia casei, Meek and Worthen. 



In 1866 Meek and Worthen proposed to separate the type of this 

 genus in a subgeneric sense from Ambonychia, under the name Megaptera. 

 This name, however, cannot stand having been used previously in zoology 

 by Gray. The preoccupation of their name having been brought to the 

 notice of Mr. Meek he subsequently (loc. cit.) suggested that if the name 

 Megaptera could not stand it might be replaced by Opisthoptera. Had he 

 characterized the subgenus and adopted the latter name himself we would 

 have no warrant to reject it, even when the rank of the group is raised 

 to that of a genus. But he did neither, and consequently the group, 

 whether viewed as a subgenus or genus, failed of being established. 

 Still, as the substitution of a new name might lead to undesirable con- 

 troversy and perhaps create unnecessary confusion, I have decided to 

 adopt Opisthoptera and to define the name as that of a distinct genus. 



As intimated in the above description Opisthoptera comprises two 

 groups of species. The typical section is restricted to the upper beds of 



