Lamellibranchiata. 319 



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. 



Ptychoptbeia Eudora. 



Ptychopteria Eudora, Hall. Pal. N. Y., vol. v, pt. 1, Unpublished. 



Shell of medium size, rhomboidal; body 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 long, 

 shallow sinus; base broadly curved; posterior end acutely recurved. 

 Left valve gibbous above, convex below. Eight valve unknown. 

 Hinge-line straight, greater than the height of the shell. Beak a 

 little in front of the anterior third of the hinge, directed forward, 

 rising above the cardinal line. Umbo narrow and gibbous, sub- 

 tending a very acute angle. Anterior end small, limited by a well- 

 marked and oblique byssal depression ; extremity acute. The dis- 

 tance from the byssal sinus to the cardinal margin is one-half the 

 greatest height of the valve. Wing joining the body one-fourth its 

 length above the posterior end ; the shallow furrow and fold defin- 

 ing its limits are not strongly marked ; margin obliquely truncate ; 

 extremity not produced. 



Test thin, marked by radii which are very fine and undulating 

 on the body and wing, while they are obsolete on the anterior por- 

 tion ; these are cancellated by fine concentric striae, which on some 

 portions are fasciculate and very conspicuous on the anterior end. 



The specimen described has a length of 36 mm., height 20 mm., 

 and hinge-line about 23 mm. 



This species bears considerable resemblance to P. Eucrate, but its 

 anterior end is proportionally narrower, and it may also be distin- 

 guished by the absence of a continuous angularity along the body, the 

 more abrupt recurving of the post-basal margin, the more oblique 

 truncation of the wing, and the less conspicuous surface markings. 

 It differs from P. Spio in its more gibbous umbo and more elevated 

 beak, narrower anterior end, more abrupt recurving of the posterior 

 margin, and conspicuous byssal sinus. 



Formation and locality. In a conglomerate of the Chemung group 

 at Panama, Chautauqua county, N. Y. 



