324 Thirty-fifth Report on the State Museum. 



hinge-line 36 mm. A right valve in the same association has a 

 length of 40 mm., height 20 mm., and hinge-line 33 mm. 



This species somewhat resembles P. irigonalis, but the posterior 

 extremity is more produced, and the wing more obliquely truncate on 

 the margin. The right valve in general aspect resembles the right 

 valve of P. alata, but the left valve is very dissimilar. The distin- 

 guishing characters are its large rhomboidal form, extreme gibbosity 

 and arcuation of the left valve. 



Formation and locality. In a compact sandstone of the Upper 

 Chemung group, in the lower beds as seen at Warren, Pa., 



Ptychopteria spatulata. 



Ptychopteria spatulata, Hall. Pal N. Y., vol. v, pt. 1. 



Shell large, elongate, rhomboidal; body sub-elliptical, oblique at an 

 angle of about 30° with the hinge ; length more than one-third 

 greater than the height ; ante-byssal margin oblique, gently curv- 

 ing into the sinus, thence extending into the broadly curved basal 

 margin; posterior end rectangularly or acutely recurved. Left 

 valve moderately convex, slightly gibbous above. Right valve 

 depressed-convex, more sub-angular along the post-cardinal slope, 

 and smaller than the left. Hinge-line straight, length about one- 

 seventh less than the greatest length of the valve. Beaks at about 

 the anterior fourth, prominent and incurved over the hinge-line in 

 the left valve; depressed in the right valve and not rising above the 

 cardinal line. Umbonal angle nearly rectangular. Anterior end 

 large, limited by a broad, slightly oblique, undefined byssal depres- 

 sion; extremity acute and nasute. Wing elongate, triangular, 

 joining the body at the posterior end; the dividing furrow distinct 

 in the left valve, more conspicuous in the right; the fold is not 

 defined in the left valve, and is strongly marked in the right ; 

 margin :nearly vertically truncate ; extremity not perceptibly pro- 

 duced. 



Test thin, marked by fine radii, which are conspicuous upon the 

 body and wing, and obsolete on the anterior end and byssal depres- 

 sion. The surface is also marked by fine concentric striae, which 

 are crowded into fascicles, giving an apparent undulation to the 

 surface from the umbo to the lower extremity, especially in the 

 right valve. At the post-basal margin the striae are recurved at a 

 little less than a right angle in the left valve, and more acutely in 

 the right valve, and they are strongly lamellose on the anterior end. 



