320 Thirty-fifth Keport on the State Museum. 



Pttchopteria alata. 



Ptychopteria alata, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 

 PI. 23, figs. 25, 26 (21, 22?). Jan., 1883. 



Shell large, broadly rhomboidal; body sub-cylindrical, oblique at 

 angle of about 45° with the hinge-line; height less than two- thirds 

 the greatest length ; anterior margin oblique, sloping into the broad 

 curvature of the base ; posterior margin recurved almost rectangu- 

 larly, slightly acute. Left valve gibbous and angular along the axis 

 of the body from the umbo to the post-basal extremity. Eight 

 valve less convex, and less angular in character than the left. Hinge- 

 line straight, about one-sixth less than the greatest length of the 

 valve. Beak very prominent in the left valve, directed forward; 

 more depressed in the right valve ; situated at about the anterior 

 fifth of the cardinal line. Anterior extremity in the right valve 

 small, triangular, acute. Wing large, triangular, joining the body 

 at the posterior extremity ; the limiting furrow is obscure in the 

 left valve, but very marked in the right ; margin of wing gently 

 concave, nearly vertical; extremity angular, little produced. 



Surface marked by fine, radii which (in the specimens seen) are 

 obscure in the left valve and nearly obsolete in the right ; and by 

 concentric strise which are crowded into imbricating ridges. Some 

 small right valves found in the same association, and referred with 

 doubt to this species, preserve the radii in a much stronger degree. 



A large right valve has a length of 42 mm., height 26 mm., and 

 hinge-line 35 mm. 



This species is distinguished by the great contrast in the convexity 

 of the two valves. The left valve is strongly angular and elevated 

 from the umbo to the posterior extremity; the right valve is con- 

 spicuously alate. 



Formation and locality. In a coarse sandstone of the Chemung 

 group, near Salamanca, N. Y. 



Ptychopteria trigokalis. 

 Ptychopteria trigonalis, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell small or of medium size, rhomboidal ; body sub-cylindrical, 

 oblique at an angle of about 45 ° with the hinge-line ; length one- 

 third greater than the height; anterior margin rounded, gently 

 curving into the broad base, with a faint byssal sinus ; posterior 

 margin almost rectangularly recurved. Left valve convex, gibbous 



