LAMELLIBRANCHIATA. 133 



Anterior end sliort, small, acute, limited by a shallow byssal depression. 

 Wing large, triangular, extending to near the posterior extremity of the body ; 

 margin slightly oblique, scarcely concave ; extremity apparently not produced. 



Surface marked by fine radii which are more or less interrupted by the 

 varices of the concentric strise, and are frequently undulating. The concentric 

 striae are often fasciculate, and in their usual condition give an undulated 

 appearance to the cast, but where well-preserved they are angular and some- 

 what lamellose. 



Interior unknown, except a narrow groove along the ligamental area of 

 the hinge. 



A left valve of usual dimensions has a length of 30 mm., height 19 mm., 

 hinge-line 23 mm. A similar right valve has a length of 31 mm., height 18 

 mm., and hinge-line 22 mm. 



This species differs from P. Proio in its less angular posterior slope and 

 posterior end ; the margin of the wing is less oblique to the hinge and the 

 extremity is not produced. 



The specimens occurring in a conglomerate or coarse sandstone have been 

 subjected to different degrees of pressure and maceration, and consequently 

 present a great variety of asjiect in the surface ornamentation. The furrow 

 and fold limiting the wing are always less conspicuous in the left valve ; and 

 in some cases when the shell has suffered pressure these characters are very 

 obscure. 



Formation and locality. Abundant in a conglomerate of the Chemung group 

 at Panama, Chautauqua county, N. Y. 



Ptychopteria Eucrate. 



PLATE XXUI, KIG. '24; PLATK I.XXXV, FIGS. 27, 28. 

 Plyclniptcria Eucrate, Hall. Pal. N. Y., vol. v. pt. 1. PIhIps and Exiilanati(m.« ; PI. 'I'i, tig. 24. -Jan., l.S!S3. 



iShell above the medium size, rhomboidal ; body very narrow, elongate-ovate, 

 oblique at an angle of about 30*^ with the hinge-line ; length nearly twice 

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



