Lamellibranchiata . 261 



also not preserved on either of the ears. Ligamental area narrow, 

 appearing as a simple fold of the hinge-margin. 



The specimen has a length of 13 mm., height 11 mm., hinge-line 

 16.5 mm. 



In some of the modes of occurrence, this species resembles P. ex- 

 foliatus, but differs in its proportionally longer hinge-line, more 

 extended posterior ear, comparatively larger anterior ear, and the 

 greatest convexity is nearer the beak. 



Formation and locality. Upper Helderberg limestone, Western New 

 York. The particular locality unknown. 



Pterikopecten nodosus. 



Pterinopecten nodosus, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell small, sub-rhomboidal, slightly oblique; form not fully known. 

 Left valve very convex. Beak prominent, nearly erect. Umbonal 

 region very prominent, the limits distinct on the anterior and ob- 

 scure on the posterior side ; subtending an acute angle. Posterior 

 ear large, triangular; margin very slightly concave ; extremity 

 angular. Anterior ear not preserved in the specimen. 



Test thin, marked (in a partially exfoliated specimen) by strong, 

 rounded rays with intermediate finer ones, and fine, concentric, 

 undulating, elevated striae, with numerous concentric undulations, 

 which increase in frequency from the beak to the margin, and on 

 crossing the larger rays, are elevated into strong nodes. On the 

 posterior ear the rays are fine, equal and continuous. 



This species is described from an imperfect specimen of the left 

 valve ; but it is so remarkable in its nodose undulations that it is 

 readily distinguished from every other form. 



Formation and locality. In the Corniferous limestone at the falls 

 of the Ohio, near Louisville, Ky. 



Pterinopecten exfoliatus. 



Pterinopecten exfoliatus, Hall. Pal. N.Y., vol.v, pt. 1. Plates and Explana- 

 tions: PI. 1, figs. 16, 17. Jan., 1883. 



Shell of less than medium size, sub-rhomboidal ; body obliquely ovate ; 

 length somewhat greater than the height ; pallial margins regularly 

 rounded from the anterior ear to the base of the posterior slope, from 

 whence the margin is nearly straight or slightly concave to the ex- 



