Lamellibranchiata. 341 



concentric lamellae. In this species the concentric striae do not pro- 

 duce the folds and undulations which are characteristic of L. Rafin- 

 esquii and L. Greeni. 



Formation and localities. In shales of the Hamilton group at 

 Pratt's falls, Onondaga county, and in the coarser beds at Schoharie 

 N. Y 



Leiopteria Mitchelli. 



Leiopteria Mitchelli, Hall. Pal. N. Y. , vol. v, pt. 1. Plates and Explana- 

 tions : PL 20, fig. 8. Jan., 1883. 



Shell large, sub-rhomboidal ; body ovate, moderately oblique ; height 

 greater than the length ; anterior margin nearly vertical from the 

 ear for about half the height, then broadly curving to the post-basal 

 side, thence more abruptly bending, making the posterior end 

 slightly extended. Left valve gibbous on the umbo, sloping from 

 highly convex above to moderately convex below. Eight valve 

 unknown. Hinge-line straight, equal to the length of the valve. 

 Beak anterior, acute, inclined forward, prominent. Umbonal 

 region prominent, subtending an acute angle. Ear short, limited 

 by an undefined sulcus and shallow byssal sinus. Wing large, 

 triangular, nearly flat, defined by the retral curving of the lamellose 

 striae ; margin moderately concave ; extremity abruptly acute. 



Test thin, marked by regular concentric lines of growth, with 

 distant, lamellose striae, which are raised into thin, sharp elevations 

 corresponding with the concentric folds or undulations of the shell. 

 The cast shows only the undulations. On the anterior margin and 

 wing, the lamellae are closely crowded, producing a varicose appear- 

 ance. They are more closely arranged upon the wing than on the 

 valve, making a gentle retral curve and becoming very conspicuous 

 on the hinge-margin. Interior unknown. Ligamental area narrow. 



A left valve has a length of 46 mm., height 49 mm., hinge-line 

 about 47 mm. 



This species, in general proportions, resembles A, Bigsbyi; it differs 

 in the larger and more extended posterior end, with striae nearly ver- 

 tical or gently curving, except at the hinge margin; the body wider 

 above, and the anterior basal margin less convex. With a single ex- 

 ception, the specimens of L. Bigsbyi occur in arenaceous beds, while 

 this species is common both to the softer shales and in the coarser 

 beds. 



Formation and localities. In soft shales of the Hamilton group, 

 from loose masses of rock at the south end of Seneca lake, and in the 

 coarse grits, at Schoharie, N". Y. 



