LAMELLIBRANCHIATA. 147 



Ptvchopteria perlata, n. sp. 



PLATE LXXXV, FIGS. :». 39. 



Shell large, rhomboidal ; l>0(ly ovate, oblique ; length one-sixth greater than 

 the height ; anterior and basal margins broadly curved ; posterior end angular. 



Left valve very convex, gibbous above. Right valve somewhat gibbous 

 above, having a convexity of about one-half that of the left valve. 



Hinge-line straight, nearly equal to the length of the shell. 



Beak at about the anterior third of the cardinal line, prominent and 

 arching over the hinge in the left valve. Umbo gibbous in the left valve 

 and convex in the right, subtending an acute angle. 



Anterior end large, obscurely defined by a shallow byssal depression, which 

 is obsolete in the right valve ; margin convex ; extremity acuminate. Wing 

 broad triangular, joining the body at the posterior end ; the furrow and fold 

 are stronger in the left valve ; margin slightly convex below, inclining a 

 little forward ; extremity produced, acute-aiigular. 



Surface of the left valve marked by fine, close, rounded radii, which are 

 often interrupted by the concentric lamellae. The radii are finer upon the 

 wing, obsolete upon the anterior end, and scarcely present on the right valve. 

 The concentric lines of growth and Limellie, give an irregularly undidating 

 appearance to the surfiice. 



Interior unknown. 



A left valve has a length of 31 mm., height 26 mm., and hinge-line oU nnn. 

 A right valve measures 33 mm. in length, 27 mm. in height, and hinge-line 

 32 mm. 



This species is a shorter, more erect form than P. lata, and the left valve is 

 more convex and broader below. 



Formation and localifij. In the upper beds of the Chemung group at 

 Warren, Pa. 



