194 PALEONTOLOGY OF NEW YORK. 



Leptodesma Stephanf, n. sp. 



PLATE LXXXIX, FIGS. iO-.'-J. 



SiiKLi, abuvf 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 

 al)ove, 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 gibl)Ous at the umbo. 



Hinge-line straight, equal to al)()ut 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 tlian 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 abru])tly 

 recurved and produced into a spiniform extension. 



Test of moderate thickness, marked by nearly equidistant lamcllose 

 concentric stria\ witli intermediate finer strite of growth. The lauicllose 

 strife are sul)du(>(l upon the wing, and very strong and irregular on the 

 anterior side. 



Interior unknown. 



A large specimen of this species has a length of 4;! mm., height 2-3 iiiiu., 

 and hinge-line about 33 mm. A somewhat smaller example lias a length 

 of 37 nnn., height 2-5 mm., and hinge-line -Kl mm. A right valvi' measures 

 33 mm. in Icngtli. 24 iiiiii. in lieight, and hinge-line 2S mm. 



Tliis .sjjccies resembles L. Matheri, with which it is associated, l)iit ilifVers in 

 its greater obliquity, more elongate outline, more extended and llatler wing, 



