LAMELIJBRANCHIATA. 129 



P T Y C II P T E R I A , Hall. 

 Ptychopteria Proto. 



PLATE XXIII. FIGS. 12, 14. 



Plychopteria Proto, Hall. Pal N. Y., voL v, pt. 1. Plate.-^ luul Explanations: PI. 2B, tigs. 12, 14. 



.Jan., 1S83. 



Shell small, rhomboidal ; liody narrow-ovate, oblique at an angle of about 35° 

 with the hinge-line ; length more thiin one-third greater than the height ; 

 ante-byssal margin sub-truncate, rounding into the broad sinus ; base broadly 

 rounded ; posterior end somewhat abruptly curved. 



Left valve regularly convex below, gibbous in the middle and above. 

 Right valve unknown. 



Hinge-line straight, length a little greater than the height of the valve, 

 and nearly equal to two-thirds its length. 



Beak at about the anterior third of the hinge, prominent, inclined forward. 

 Umbo abruptly gibbous, subtending an ticute angle. 



Anterior end large,* rounded, limited by a broad byssal depression directed 

 backward ; extremity angular. Wing small, narrow-triangular, extending 

 to near the posterior end of the liody ; margin scarcely concave ; extremity 

 obtuse. 



Surfiice marked by fine, regular, elevated radii, which are less conspicuous 

 on the wing ; also by concentric striaj, which are often crowded and lamel- 

 lose, producing a sonjewhat undulated character of the sui^face. 



Interior unknown. 



A large left valve has a length of 2!t mm., height 18 mm., and hinge-line 

 20 mm. A smaller specimen has a length of 20 mm., height lo mm., and 

 hinge-line I 4 mm. 



This species resembles P. sinuosa, but the anterior end is wider ; the wing 

 is smaller and not produced at the -extremity , and the radii are coarser. These 



*In the descrijitions of the species of Ptychopteria ami Leptodesma, the term aiiricli' or eai- is not so 

 applicable to the anterior extension as in Actinoptei'ia and Tjeiojiteria and the tiM-ni anterior end has Leen 

 used for that portion of the shell anterior to the liy.ssal sinns. 



17 



