LAMELLIBRANCHIATA. 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 l)yssal sinus ; thence nearly 

 straight to within one-fourth the length of the valve from the posterior 

 extremity, which is regulaidy and broadly rounded. 



Left valve regularly convex below, narrow and gibbous above. Right 

 valve depressed-convex below, more convex above, and modei'ately 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 giblwus, 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, tei^minating in a short spiniform extension. In the right valve the 

 wing is proportionally larger and less defined. 



Test thick, mai'ked upon the body and wing with sharp, elevated, 

 lamelliform strige and intermediate finer strige ; on the anterior the striae are 

 fasciculate. 



Interior unknown. 



A large left valve has a length of 40 nun., height 20 mm., hinge-line 32 

 mm. A right valve has a lengtli 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 striag are 



