LAMELLIBRANCHIATA. 213 



Leptodesma Jason, n. sp. 



PIRATE \' I. 1 [GS. I 6. 



Shell above the medium size, sub-triangular; body elongate-ovate or Bub- 

 spatulate, attenuate toward the anterior end, oblique at an angle of aboul 



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 stria? 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 

 nun. 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 win- 

 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 stria- are 



