140 PALEONTOLOGY OF NEW YORK. 



This species is (listinj^uislicd hy the great contrast in the convexity of the 

 two valves. Tlie left valve is strongly angular ami elevated iVoin the uniho to 

 the posterior i-xtremity : tlu' right valve is conspicuously alate. 



Formation and locality. In a coarse sandstt)ne of the Chenunig group, near 

 Salamanca. X. Y. 



Ptychopterta trigonalis, u. sp. 



PLATE I, XXXV, FIGS. 34, 35. 



Shell small or of medium size, rliomhoidal ; liods siilj-rylinih-ical, (ililiijue at an 

 angle of about 4')° with the hinge-line ; length one-third greater than the 

 height; anterior m;n;j.iii lounded, gently curving into the broad base, with 

 a faint byssal sinus; postei'ior margin almost rectangularly recurved. 



Left valve convex, gibbous on the umbo, and angular fi-om the umbo to 

 the post-basal extremity. Right valve uidsnown. 



Hinge-line straight, length greater than the height of the valve. 



Beak in iront of the anterior third of the hinge, obtuse, prominent and 

 incurved. Umbonal region gibbous, sul)tending an acute angl(\ 



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 oldei- examjiles; margin somewhat obliquely truncate; 

 extremity not produced. 



Surlace marked by fine radii, wiruh are obscure or obsolete on the anterior 

 part of the shell ; also by concentric strise, which are somewhat acutely 

 ri'curved on the angular portion of the body, and in older shells art' fascicu- 

 late, producing an undulated aspect. 



Interior unknown. 



A small left valve has a length of It; mm., height I I mm., and hinge-line 

 13 mm. Another specimen has a length of 30 nun.. Iieight 18 mm., and 

 hinge-line 25 mm. 



