LAMELLIBRANCHIATA. 149 



PtYCHOPTERIA GIBBOSA, 11. sp. 



PLATE LXXXV, FIG. 5. 



Shell small, rliomboidal ; body broad-ovate, oblique at an angle of about 45° 

 with the hinge-line ; length not more than one-seventh greater than the 

 height ; ante-byssal margin oblique, curving into the marked sinus, thence 

 along the base to the nearly rectangularly recurved posterior extremity. 



Left valve very convex, gibbous on the umbo and in the middle. Right 

 valve unknown. 



Hinge-line straight, nearly as long as the length of the shell. 



Beaks a little anterior to the middle of the hinge ; very prominent in the 

 left valve and arching over the hinge. Umbonal region ample, very gibbous, 

 obscurely angular along the posterior slope, subtending an acute angle. 



Anterior end somewhat large, gibbous, and separated from the body of 

 the shell by a nearly vertical sulcus ; extremity acute and nasute. Wing of 

 medium size, joining the body at the posterior end ; the furrow and fold are 

 very conspicuous, the fold somewhat broader and nearly equal to the portion 

 of the wing above ; margin almost vertically truncate ; extremity apparently 

 not produced. 



Test thin, marked by fine radii and concentric striae. The concentric 

 lines are crowded into fascicles, producing ao undulated appearance ; and are 

 strongly lamellose on the anterior end ; the radiating striaB are distinct upon 

 the body of the shell, obscure on the wing, and obsolete on the anterior end. 



Interior unknown. 



A specimen of the left valve has a length of 26 mm., height 15 mm., 

 hinge-line 22 mm. Another has a length of 20 mm., height 13 mm., hinge- 

 line 18 mm. 



This species is distinguished by the sub-central position of the beak, and the 

 great prominence of the umbo ; the marked byssal depression, and strong 

 furrow and fold of the wing. 



Formation and locality. In the upper beds of the Chemung group at Warren, 

 Pa. 



