130 G. 0. SARS. CRUSTACEA. [norw. pol. exp, 



allied species C horealis, where it assumes in some places an imbricated, 

 squamous character. At the upper posterior corner there are 3 or 4 successive 

 small denticles, somewhat increasing in size posteriorly. These denticles are, 

 however, as a rule, only present on the right valve. The free edges of the 

 valves are perfectly smooth throughout their whole length. 



The animal is enabled to withdraw itself completely into the shell; but 

 more generally the tip of the frontal tentacle, the terminal appendages of 

 the antennulae, and the natatory ramus of the antennae are seen projecting 

 in front from the above-mentioned opening, and below, the tip of the man- 

 dibular palps and the caudal plates, as a rule, also project beyond the edges. 

 The animal is fixed to the shell by a strong adductor muscle joining each 

 valve at about the centre, and just above this muscle, it is suspended to 

 the dorsal face by a comparatively short ligament, within which the heart 

 has its place. We may distinguish 2 chief divisions of the body, an anterior 

 or cephalic part, and a posterior or abdominal part, both defined by the 

 above-mentioned dorsal ligament and by the adductor muscle. The anterior 

 division is, as it were, cut off in front, even being somewhat concave in its 

 upper part, whereas below it projects into the hood-like anterior lip. It 

 carries the antennulee above, and between them a very delicate tentacular 

 appendage. The enormously developed antennae are attached to the sides, 

 and below, the mandibles with their palps, and the maxillae originate. The 

 posterior, or abdominal division is very voluminous, and freely mobile within 

 the hollow of the shell. It is covered by a soft skin closely wrinkled trans- 

 versally, and is deflexed, exhibiting dorsally, at about the middle, 2 successive 

 short prominences. Below, this division carries 3 pairs of legs, and to the 

 gradually tapering and somewhat anteriorly curving end, the coarsely spinous 

 caudal plates are secured. 



The frontal tentacle (see fig. 4) is very delicate, extending as a narrow 

 rod straight in front, and terminating in a slightly dilated, oblong fusiform, 

 and somewhat deflexed capitulum, which is finely hispid throughout, and 

 projects just beneath the rostral prominence of the shell. 



The antennulae (ibid) are likewise of rather delicate structure, and, it 

 would seem, are scarcely mobile. They each form a simple stem extending 

 anteriorly, at each side of the frontal tentacle, and about equalling in length 



