Lamellibranciuata. 271 



low sinus; extremity acute. In the right valve sub-triangular; 

 margin rounded, defined by an abrupt sulcus and a deep, angular 

 byssal notch, which encroaches on the body of the valve. In the 

 posterior ear the extremity is obtuse in the left valve, somewhat 

 more pointed in the right valve. 



Test thin, marked by numerous fasciculate rays, which increase 

 by interstitial addition, with interspaces varying from nearly equal 

 to several times the width of the rays ; cancellated and sometimes 

 crenulated by elevated, lamellose stride of growth ; and by from 

 three to ten strong, concentric undulations, which are rounded upon 

 the summits. The markings on the ears are similar to those on the 

 body of the shell ; the rays on the anterior ear are sometimes ob- 

 scured by the striae of growth. Ligamental area narrow. 



A left valve has a length of 51 mm., height 36 mm., hinge-line 

 36 mm. A right valve has a length of 44 mm., height 3? mm., 

 hinge-line 35 mm. A young specimen has the following dimensions 

 ^ length 23 mm., height 19 mm., hinge-line 20 mm. 



This species is distinctly characterized by the strong, concentric 

 wrinkles, the deep byssal sinus of the right valve, and by the form of 

 the shell. Specimens vary in the frequency and elevation of the radii, 

 and in the number and prominence of the concentric undulations. 

 The specimens are casts of the interior, with scarcely any remaining 

 test. The surface markings are therefore subdued, and the concentric 

 striae are often not at all preserved. This species is widely distributed. 

 It has been found in the Corniferous limestone at a single locality ; 

 and in the soft shales of the Hamilton group throughout the western 

 portion of the State. 



Formations and localities. In the Corniferous limestone at Clarence 

 Hollow, Erie county, and in shales of the Hamilton group at numerous 

 localities in Central and Western New York. 



Ptekinopecteh (Ayiculopectek ?) STRICTUS. 



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

 tions : PI. 24, fig. 2. Jan., 1883. 



Shell of medium size, transversely semi-elliptical, not oblique ; body 

 of the shell ovate, length one-sixth less than the height ; anterior 

 and basal margins rounded ; posterior margin straight. Left valve 

 moderately convex. Eight valve flat. Hinge-line straight, equal 

 to the length of the shell, central as regards the transverse axis. 

 Beaks obtuse, low, directed slightly forward, anterior to the middle 

 of the hinge-line. Umbonal region moderately prominent in the 



