14 



G. 0. SARS. 



[No. 8. 



racter not found in either of the northern species, and which 

 therefore gave rise to the specific denomination. A fourth spe- 

 cies of the genus, D. angastum, has been described by Dana, 

 but the description and figures are so irnperfect as to preclude 

 any concise definition of the specific characters. 



Description of the Female. — The form of the body 

 (see Pl. 2, figs. 1, 2), as in the other known species, rather elon- 

 gate, with the height and breadth about equal. 



The head, occupying about 1 I% of the total length, is com- 

 paratively narrow and, as usual, without the slightest trace of 

 either rostrum or fornix. It is marked off from the carapace by 

 a distinct dorsal impression, at the bottom of which a trans- 

 versal suture passes to either side, joining the upper end of the 

 mandibles. Viewed from the side (see fig. 2), the head exhibits 

 a somewhat oblongo-quadrilateral form, being obtusely truncate 

 anteriorly and having the dorsal margin slightly arched, whereas 

 the ventral appears almost straight, with but a very slight con- 

 vexity in the middle, and not presenting a trace of the frontal 

 sinus immediately beneath the eye, so distinctly marked in D. 

 brachyurum. In a dorsal or ventral aspect (see fig. 1) the head 

 appears obtusely conic, with the front part rounded off. 



The so-called shell, or more properly the carapace, exhibits 

 a form very similar to that in the two northern species, being 

 oblong-oval and abruptly truncate posteriorly, with the upper 

 angle somewhat projecting and well-nigh a right one, whereas 

 the lower is obsolete. It appears more or less tumid, and ex- 

 hibits a varying dorsal convexity, according to the number 

 of ova, or embryos, contained within the incubatory cavity, or 

 matrix; when these are very numerous, a slight sinus is formed 

 immediately in front of the postero-superior angle by the bulging 

 out of the dorsal face. The free parts of the carapace, or 

 valves, occur comparatively small and narrow, being deeply 

 emarginate anteriorly, and thus leave all of the oral appen- 

 dages uncovered. Seen in a lateral aspect (see fig. 2), their 

 ventral margins appear almost straight, forming at the antero- 

 lateral corners a sharp curve, whereas at their junction with 



