LAMELLIBRANCHIATA. 209 



Leptodesma Hector, n. sp. 



PLATE XC, FIGS. IH. 20, 



Shell of medium size, sub-rhomboidal ; body narrowl}' elongate-ovate, oblique 

 at an angle of about 30° with the cardinal line ; length a little more than 

 once and a half the height ; ante-byssal margin oblique, carving into a broad, 

 shallow sinus ; ventral margin broadly curved ; posterior margin abruptly 

 recurved. 



Left valve moderately convex below, narrowly gibbous above. Right 

 valve depressed below, gently convex above. 



Hinge-line straight ; length equal to about four-fifths of the length of tlie 

 valve. 



Beaks sub-anterior, directed forward, acute, slightly elevated al)ove the 

 hinge. Umbonal angle very acute. 



Anterior end narrow, small, produced into an acute extension, defined by 

 a shallow byssal depression. Wing large, joining the body at three-fourths 

 or more of its length from the ))eak ; margin very oblique below, deeply 

 sinuate just- beneath the cardinal line, turning abruptly backward and 

 terminating in a spiniform extension. The wing of the right valve is 

 scarcely defined, and the shell has a somewhat broadly spatulate aspect. 



Test thin, marked by line, close, concentric stria^, and, as usually seen, 

 the surface shows nearly equally distant lamellose stria? on the body and 

 wing, which are crowded on the anterior side and extension. In the right 

 valve these characters are more subdued on the body than on the wing. In 

 ordinary conditions, the specimens mostly exhibit only oljscure indications 

 of the striation of the shell. 



Muscular impression in the right valve large, sub-circular, placed below 

 the middle of the length of the posterior slope. 



A left valve, of the usual size, has a length of 35 mm., height 20 mm., 

 hinge-line 27 mm. A small specimen has a length of 20 mm., height 12 

 mm., and hinge-line 1(1 mm. Young shells of this species are more gibbous 

 than the older examples. 

 27 



