LAMELLIBRANCHIATA. 217 



Leptodesma nereus, n. sp. 



PLATE XC, FIGS. 31-:53. 



Shell of medium size, sub-rhomboidal ; body broadly ovate, narrowing rapidly 

 to the anterior end, and oblique to the hinge-line at an angle of 45° ; length 

 less than one-third greater than the height ; ante-byssal margin gently curved, 

 extending into a wide sinus, broadly rounded along the base, curving regularly 

 on the post-basal side and abruptly recurved in the upper part. 



Left valve gently convex below, more convex above, and moderately 

 gibbous on the umbo. Right valve smaller, more depressed below, nearly 

 equally convex toward the umljo. 



Hinge-line straight, less than the length of the valve. 



Beaks sub-anterior, acute, directed forward, rising slightly above the 

 hinge-line. Umbonal region gibbous in the left valve, narrower but equallv 

 gibbous in the right valve. Umbonal angle acute. 



Anterior end small, abruptly curving into the byssal depression ; extremity 

 acute, not prolonged. Wing broad, extending along the body about four- 

 fifths of its length from the beak ; margin very oblique in the lower part 

 and moderately concave above the centre, then turning abruptly backward 

 just below the cardinal line, and produced into a short spiniform extension. 

 In the right valve the wing is less defined and less concave on the margin. 



Test thin, marked by concentric stride, which are fasciculate on the body 

 and anterior part, and more elevated, distant and sharp upon the wing. 



Muscular impression large, sub-circular, situated on the posterior slope 

 below the middle of the length. The pallial line extends from the lower 

 part of the muscular impression nearly parallel to the anterior and basal 

 margin. The hinge of the left valve has a strong groove extending its entire 

 length ; the right valve has an oblique posterior fold. 



One specimen has a length of 35 mm., height 26 mm., hinge-line .31 mm. 

 A similar example has a lengtli of 34 mm., height 24 mm., and hinge-line 

 28 mm. A right valve has a greatest length of 31 nnn., height 20 mm., and 

 hinge-line 25 mm. 

 28 



