70 PALEONTOLOGY OF NEW YORK. 



PtERINUPKCTEN KEGULARIS, 11. sp. 



I'l.ATE LXXXIII, FIG. 1. 



Shell of medium !<ize, semi-circular, body ui" the valve oblique; leuj^th nearly 

 one-fifth greater than the height, greatest length above the middle ; margins 

 very full and regularly rounded from the byssal sinus to the posterior extrem- 

 ity of the hinge. 



Left valve moderately convex. Right valve unknown. 



Hinge line straight, anterior ; length a little greater than the length of 

 the valve. 



Beak of left valve acute, directed forward, situated anterior to the middle 

 of the hinge. 



Anterior ear small, triangidar, more than half as long as the posterior 

 ear, limited by a broad sulcus; margin convex; extremity rounded. Posterior 

 ear large and undefined from the body of the v^alve ; margin convex, in 

 regular continuation with the pallial margin; extremity obtuse, angular. 

 Byssal sinus broad. 



Test thin, marked with iiumei-ous fine, alternating, filiform, elevated striae 

 which are crossed by irregular lines of growth. The ornamentation of the 

 ears is somewhat more subdueil than on tlic body of the valve. 



Muscular impression sm.all, ovate, situati^d below the miildlc of the post- 

 cardinal slope. 



A specimen of the left valve has a length of 2-4 mm., height 19 mm., 

 hinge-line 2-") mm. A larger example measures 30 mm. in length. 



This species is distinguished for the regular continuous convex curvature of 

 the outline from the byssal sinus to the posterior extremity of the hinge. In 

 this respect it differs from any other form of tliis genus here described. The 

 right valve of P. filitextus has a similai- outline, but the left \nl\-e has l)een 

 shown to have the margin of the jjosterior ear concave and the extremity 

 acute-angular. The specimen of the right valve of that species, figured in 



