I'M PALAEONTOLOGY OF NEW YORK. 



Leptodesma Stepham, n. sp. 



i'i. vn: i. xxxix. figs so as 



Shell above the medium size, sub-rhomboidal ; body ovate, straight on the post- 



umbonal slope, oblique at an angle of from 30° to 40° with the binge-line; 

 height about five-eighths of the length; ante-byssal margin vertically truncate 

 above, curving below into the sinus: basal margin broadly rounded, abruptly 

 recurved ai the post-basal extremity. 



Left valve convex, somewhat gibbous above. Right valve depressed- 

 convex below and abruptly gibbous at the umbo. 



Hinge-line straight, equal to about five-sixths the length of the valve. 



Beaks sub-anterior, directed slightly forward, obtuse, prominent, rising 

 a little above the cardinal line. Umbonal region regularly gibbous, subtending 

 an acute angle. 



Anterior end huge, short, defined by a nearly vertical byssal depression, 

 which reaches the margin about one-third tbe length of tbe valve from the 

 anterior extremity, which is abruptly rounded or truncate. Wing joining the 

 body at less than one-fourth the length of the valve from the posterior 

 extremity: margin very oblique below, deeply concave above: the greatest 

 depth of the concavity is above the middle, whence the margin is abruptly 

 recurved and produced into a spiniform extension. 



Test of moderate thickness, marked by nearly equidistant lamellose 

 concentric stria', with intermediate finer striae of growth. The lamellose 

 striae are subdued upon the wing, and very strong and irregular on the 

 anterior side. 



Interior unknown. 



A large specimen of this species has a length of 43 mm., height 25 mm., 

 and hinge-line about 33 mm. A somewhat smaller example has a Length 

 of -'17 mm., height 2-"> mm., and hinge-line 30 mm. A righl valve measures 

 ■'!•'! mm. in length, - I mm. in height, and hinge-line 28 mm. 



This species resembles L. Matheri, with which it is associated, but differs in 

 it- greater obliquity, more elongate outline, more extended and flatter wing, 



