Lamellibraxchiata . 263 



it differs in surface markings and depth of byssal notch, and is ap- 

 parently thinner and more delicate. Hinge-line straight, length 

 equaling, or greater than, the greatest length of the shell, extend- 

 ing anteriorly beyond the margin of the valve. Beaks obtuse, 

 rounded, anterior to the center of the hinge-line, directed slightly 

 forward. Umbonal region ample, subtending an acute angle. Ears 

 triangular, flat. The posterior ear larger and less distinctly defined 

 than the anterior; margin straight or slightly concave; ex- 

 tremity rectangular. Anterior ear rounded ; margin convex ; 

 extremity acute-angular. Byssal sinus moderate in the left valve ; 

 deep, narrow and angular in the right valve. 



Test of the left valve marked by numerous strong, rounded, in- 

 creasing radii, with broader interspaces, containing one or two 

 smaller rays which originate about the middle of the valve ; crossed 

 by fine, sharply elevated, concentric striae. The right valve is marked 

 by fine, sub-equal, regular, radiating striae, which increase by inter- 

 stitial addition, and become obsolete on the upper part of the 

 umbo. The radii are also seen on the ears. 



The largest specimen has a length of 19 mm., height 15 mm., 

 hinge-line 19 mm. Another specimen measures 17 mm. in length, 

 14 mm. in height, with the hinge-line 18.5 mm. 



This species closely resembles P. Iwtus, but differs in being less 

 oblique, with greater height, more acute beak, fewer and stronger 

 radii, and a characteristic concentric marking. From P. Hermes it 

 differs in being less oblique, with the posterior ear less extended, and 

 with finer rays upon the ears. From P. conspectus it differs in its 

 greater length, and distinctly different surface markings of the left 

 valve. 



Formation and locality. In the Marcellus shale at Bloomfield, 

 Ontario county, N. Y. 



Pterinopectek l^tus. 



Pterinopecten lautus (in error), Hall. Pal. N". Y., vol. v, pt 1. Plates and Ex- 

 planations : PI. 1, fig. 13. Jan., 1883. 



Shell small, sub-rhomboidal, regularly rounded ; length one-sixth 

 less than the height. Left valve depressed-convex. Right valve 

 unknown. Hinge-line straight, longer than the greatest length of 

 the valve, extended anteriorly beyond the margin of the shell. 

 Beak obtuse, anterior to the middle, directed forward, not prominent. 

 Umbonal region scarcely defined, subtending about a right angle. 

 Ears unequal, triangular. Posterior ear much the larger, undefined; 



