8 G. O. SABS. [No. 8. 



above (fig. 2) the shell exhibits a very narrow oblong, or nearly 

 fusiform shape, tapering somewhat more rapidly anteriorly 

 than posteriorly. The greatest width, situated a little behind 

 the middle, is considerably less than the height and scarcely 

 exceeds 2 / 5 of the length. The anterior extremity is much 

 narrower and more pointed than the posterior, and along the 

 median line there is behind the middle a rather deep groove. 

 The shell of the adult male (fig. 3) differs very little from that 

 of the female, being perhaps a little shorter in proportion to its 

 height and somewhat more expanded in its posterior part. 



The valves are in both sexes rather thin and pellucid, with 

 a smooth, shining surface and sparingly beset with very fine 

 hairs, more distinct at either extremity. The inner duplicatures 

 of the shell, extending inside the free edges of the valves (see 

 PL IV, figs 1 and 2), are very narrow, though a little broader 

 on either extremity than ventrally. In the ventral part, an- 

 terior to the middle, the edges are, as usual, to a short extent 

 inflexed, dorsally they are connected by an elastic band, but 

 without any distinct hinge-teeth. The two valves are rather 

 unequal, the left being much the larger and distinctly overlap- 

 ping the right at both extremities as also along the ventral 

 margin. Moreover the right valve (fig. 2) is prominently distin- 

 guished by its free edge being in the greater part of its extent 

 armed with a regular series of small, knobshaped teeth, giving 

 it a peculiar crenulated appearance (see fig. 3). On the left 

 valve (fig. l) there is no trace of such teeth, whereas it exhibits 

 at both extremities a rather broad and extremely thin and 

 hyaline border, also observed on the right one, but here con- 

 siderably narrower. The muscular impressions of the great adduc- 

 tor of the shell are distinctly seen in the middle part of either 

 valve, both outside and inside, as an assemblage of lucid spots, 

 two of which are situated at some distance in front of the others 

 and forming together an oblique line (see PL IV, fig.[2). 



In quite young specimens (PI. I, fig. 4) the shell, as usual, 

 exhibits a form rather differing from that in adults, the anterior 

 part being much higher than the posterior, which is almost 



