648 GEOLOGY OF OHIO. 



Opisthoptera notabilis, n. sp. 



Plate 49, Fig-. 16. 



Shell very elongate for this family, highest posteriorly, the length 

 a nd height respectively as sixteen is to nine. Cardinal margin long, 

 nearly straight, rather acutely prolonged posteriorly though not as far as 

 the rounded part of the posterior outline; basal margin broadly convex, 

 ascending anteriorly, turning abruptly into the short, nearly straight, and 

 almost vertical anterior margin. Beaks neither large nor prominent; 

 valves with the greatest convexity in the anterior third, the anterior slope 

 rapid though not abrupt; wing compressed; byssal opening apparently 

 long and very narrow. Surface marked with distinct but irregular con- 

 centric lines of growth. Excepting the compressed cardinal slope or 

 wing, on which radiating costse are very obscure or are wanting entirely, 

 the rest of the surface is thrown into about twelve radial folds, of which 

 the five or six occupying the central part of the valves are large and 

 moderately prominent. The remainder, however, gradually decrease in 

 size and distinctness anteriorly. 



Though clearly a near relative of O. latico statu, the present species is 

 still not at all likely to be confounded with it nor with any other ambony- 

 chioid shell now known. The distinctive features are the unusual length 

 of the shell and the comparative obscurity of the costse. 



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

 cinnati, Ohio. 



Genus PSILONYCHIA, n gen. 



Shell large, slightly oblique, high, acuminate-ovate; anterior side 

 abrupt, beaks prominent, incurved, terminal. A large byssal opening in 

 the somewhat flattened upper half of the anterior side. Surface marked 

 with concentric lines of growth only. Hinge rather short, scarcely alate 

 posteriorly, apparently without teeth; ligamental area well developed, 

 horizontally striated. Muscular scars and pallial line as in Byssonychia 

 and other genera of this family. 



Type : P. perangulata, n. sp. 



This interesting genus seems to occupy an intermediate position 

 between the radially ribbed genera Byssonychia, Eridonychia, Allonychia, 

 Anomalodonta and Opisthoptera, and the concentrically marked genera 

 Clionychia, Mytilarca, Hall, and Plethomytilus, Hall, having a large byssal 

 opening like the former and concentric surface markings only like the 

 latter. Still, we cannot regard Psilonychia as in any way lessening the 

 gap between these two groups of genera. Should species of this type 

 be discovered in, say the Chazy limestone, we might reasonably view the 

 genus as a connecting link. But, why speculate on species that probably 

 never existed? With our present light we are obliged to accept Psilony 



