LAMELLIBRANCHIA TA. 
213 
Leptodesma Jason, n. sp. 
PLATE XCI, FIGS. 4-6. 
Shell above the medium size, sub-triangular; body elongate-ovate or sub- 
spatulate, attenuate toward the anterior end, oblique at an angle of about 
40° with the hinge-line ; length once and a half the height; margin of the 
anterior end gently curving into the undefined byssal sinus; thence nearly 
straight to within one-fourth the length of the valve from the posterior 
extremity, which is regularly and broadly rounded. 
Left valve regularly convex below, narrow and gibbous above. Right 
valve depressed-convex below, more convex above, and moderately gibbous 
in the umbonal region. 
Hinge-line straight, somewhat less than the greatest length of the body. 
Beaks sub-anterior, acute, directed forward, slightly elevated above the 
hinge-line. Umbonal region abruptly gibbous, subtending a very acute angle. 
Anterior end short. Wing joining the body near the posterior extremity ; 
margin regularly concave, the greatest concavity near the middle of the 
width, terminating in a short spiniform extension. In the right valve the 
wing is proportionally larger and less defined. 
Test thick, marked upon the body and wing with sharp, elevated, 
lamelliform striae and intermediate finer striae; on the anterior the striae are 
fasciculate. 
Interior unknown. 
A large left valve has a length of 40 mm., height 26 mm., hinge-line 32 
mm. A right valve has a length of 36 mm., height 20 mm., and hinge-line 
34 mm. 
This species resembles L. Hector in general form, but is broader at the 
posterior end, more abruptly gibbous along the axis above the middle, the wing 
joins the body nearer the posterior extremity, and the margin of the wing has 
the deepest concavity in the centre of its width. The right valve is more 
convex than the right valve of L. Hector , and the concentric elevated strige are 
