NO. B.] ACCOUNT OF THE SPECIES. 129 



The general form of the shell (see PI. XXXV, figs. 1 & 2) is that charac- 

 teristic of the genus Conchoecia, being rather elongated, with the anterior 

 extremity produced above to a well-marked rostriform prominence, below 

 which, there is a distinct notch on each side. Seen laterally (fig. 1), it 

 exhibits an oblong oval, or rather somewhat cuneiform shape, being conspic- 

 uously narrowed in front, with the greatest height not quite attaining half 

 the length, and occurring in its hindmost part. The rostral prominence, in this 

 view of the shell, appears as a distinctly defined, beak-like process, projecting 

 in front, and very slightly deflexed at the tip. Immediately below it, the shell 

 has a rather deep sinus or notch, from which, in some cases, the natatory 

 ramus of the antennae may be found extended. The anterior extremity of 

 the shell, below this notch, is narrowly rounded, the margin sloping without 

 any intervening angle into the ventral one. The latter is somewhat oblique, 

 and very slightly sinuated in the middle, joining the posterior margin by an 

 abrupt cui-vature. The hind extremity of the shell appears broadly rounded, 

 or rather obtusely truncated, and forms an obtuse angle above. The dorsal 

 margin is nearly straight, and horizontal, with a very slight depression at 

 about the middle. 



Seen dorsally or ventrally (fig. 2), the shell appears moderately tumid, 

 and rather regularly oblong ovate in form, with the greatest width about 

 the middle, and somewhat exceeding Vs of the length. The lateral contours 

 are evenly cui-ved throughout, and the posterior extremity is acute, whereas 

 the anterior appears considerably broader, and is obtusely truncated at the tip. 

 The dorsal face of the shell is somewhat applanated, especially in its anterior 

 part, whereas ventrally, the valves meet at an acute angle. 



The valves are perfectly equal, and are united along the dorsal face by 

 a simple ligament, admitting of being opened to a certain extent, and again 

 closed; but anteriorly, below the rostral prominence, there always remains a 

 somewhat cordiform opening leading to the inner cavity of the shell. As to 

 consistency, the valves are very thin and elastic, and of chitinous structure; 

 and they are so pellucid, that the enclosed animal may be traced through 

 them rather distinctly. The surface is sculptured with two sets of curved stria 

 crossing each other, and producing a somewhat irregular and close reticu- 

 lation, which, however, is far from being so strongly marked as in the nearly 



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