140 
PALEONTOLOGY OF NEW YORK. 
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 conspicuously alate. 
Formation and locality. In a coarse sandstone of the Chemung group, near 
Salamanca, N. Y. 
Ptychopteria trigonalis, n. sp. 
PLATE LXXXV, FIGS. 34, 35. 
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 on the umbo, and angular from the umbo to 
the post-basal extremity. Right valve unknown. 
Hinge-line straight, length greater than the height of the valve. 
Beak in front of the anterior third of the hinge, obtuse, prominent and 
incurved. Umbonal region gibbous, subtending an acute angle. 
Anterior end short, rounded, indistinctly separated from the body by an 
obscure byssal sinus. Wing large, triangular, joining the body of the valve 
near the posterior extremity; the limiting furrow is very distinct in young 
shells, and obscure in older examples; margin somewhat obliquely truncate; 
extremity not produced. 
Surface marked by fine radii, which are obscure or obsolete on the anterior 
part of the shell; also by concentric strige, which are somewhat acutely 
recurved on the angular portion of the body, and in older shells are fascicu¬ 
late, producing an undulated aspect. 
Interior unknown. 
A small left valve has a length of 16 mm., height 11 mm., and hinge-line 
13 mm. Another specimen has a length of 30 mm., height 18 mm., and 
hinge-line 25 mm. 
