LAMELLIBRANCHIATA. 137 



Ptychopteria Spio, n. sp. 



PLATE LXXXV, FIG. 19. 



Shell of medium size, rhomboiclal ; body elongate-ovate, oblique to the hinge- 

 line at an angle of about 30° ; length considerably more than one-third 

 greater than the height ; ante-byssal margin curving into a shallow sinus ; 

 ventral margin broadly rounded ; posterior margin abruptly recurved. 



Left valve regularly and moderately convex, gilibous on the umbo. Right 

 valve unknown. 



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



Beak in front of the anterior third, moderately prominent and scarcely 

 rising above the hinge. 



Anterior extremity narrowly acute. Wing very obliquely truncate, joining 

 the body near the posterior extremity ; the furrow and fold separating it from 

 the body are only moderately developed ; extremity not produced. 



Test thin, marked by tine undulating radii which are obsolete on the 

 anterior side ; also by concentric striae which are prominent and lamellose on 

 the anterior. 



Interior unknown. 



The specimen described has a length of 35 mm., height HO mm., hinge-line 

 24 mm. 



This species differs from P. Eucrate in its comparatively greater height ; it is 

 less gibbous in the left valve, the posterior end not angularly produced, the 

 byssal depression less conspicuous, and the radii finer. It differs from P. Sao 

 in its lesser gibbosity, greater obliquity of the wing margin, and the generally 

 subdued characters of the surface markings. 



Formation and locality. In a conglomerate of the Chemung group, Panama, 

 Chautauqua county, N. Y. 

 18 



