194 
PALEONTOLOGY OF NEW YORK. 
Leptodesma Stephani, n. sp. 
PLATE LXXXlX, PIGS. 20-22. 
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 hinge-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 at 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 large, short, defined by a nearly vertical byssal depression, 
which reaches the margin about one-third the length of the 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 striae, 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 37 mm., height 25 mm., and hinge-line 30 mm. A right valve measures 
33 mm. in length, 24 mm. in height, and hinge-line 28 mm. 
This species resembles L. Matheri, with which it is associated, but differs in 
its greater obliquity, more elongate outline, more extended and flatter wing, 
