i in i'\LjEONTOlogy of new york. 



Tlii> 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 extremitj ; the right valve is conspicuously alate. 



Formation and locality. In a coarse sandstone of the Chemung group, near 

 Salamanca, N. Y. 



PtYCHOPTERIA TRIG0NAL1S, 11. Bp. 

 PLATE I.WXV. FIGS :'.4. 35. 



Shell -mall or of medium size, rhomboidal; body sub-cyhndrical, oblique at an 



angle of about 45 c with the binge-line: length one-third greater than the 

 height : anterior margin rounded, gently curving into the broad base, with 

 a faint livssal 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 hyssal sinus. Wing large, triangular, joining the body of the valve 

 near the posterior extremity ; the limiting furrow is very distinct in young 

 shells, anil obscure in older examples; margin somewhat obliquely truncate; 

 extremity not produced. 



Surface marked by tine radii, which are obscure or obsolete on the anterior 

 pari of the shell; also by concentric striae, 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 Id nun., height 11 mm., and hinge-line 

 L3 mm. Another specimen has a length of 30 mm., height 18 mm., and 

 hinge-line 25 mm. 



