19 



a. Adult Specimens (PI. I, figs. 7 a — "jc; PI. II, figs, la — id). — Very similar to 

 5. cornictilum Kr. Both species together constitute a sharply defined tribe of group I of Sergcstes. 

 Here may be given a somewhat detailed description of S. semimidus, but as it is so similar to 

 5. corniailutn that it will be difficult to point out any valid specific difference between them 

 excepting in the male petasma, it may be sufficient to refer to my figures of various parts of 

 5. cornicitlum to be found in the future paper on the Sergestidse from the French expeditions. 



The animal is slender, with a considerable distance between the moutli and the insertion 

 of the eye-stalks. The carapace is considerably compressed; its rostrum is small or even very 

 small, somewhat or only a little produced forwards, with the distal parts of the upper and the 

 lower' margin strongly converging and the end acute or produced into a minute, acute tooth. 

 A supra-ocular spine is wanting, but its crest is very conspicuous; the hepatic spine is small 

 but very distinct. The gastro-hepatic groove is deep, and from its postero-lateral angle a very 

 conspicuous keel runs forward to the antenna. The upper and the anterior margin of the 

 branchial area is limited by a deep groove, and along the upper margin runs a strong keel, 

 while the cervical groove above the front part of the branchial area is nearly or quite indistinct. 

 The abdomen scarcely affords any valuable character; its si.xth segment is a little less than 

 twice as long as deep. 



The eyes are somewhat small, conspicuously shorter but much broader than the slender 

 and a little elongate distal joint of their stalks; on the inner margin of the stalk a little behind, 

 the cornea is seen a rounded protuberance which sometimes is about as high as broad, most 

 frequently lower than broad. The antennular peduncles are long, about two-thirds as long as 

 the dorsal line of the carapace; first joint has its distal half narrower than in the preceding 

 species; second and especially third joint is slender; third joint is considerably shorter than the 

 first, somewhat longer than the second, and slightly thicker in the male than in the female. 

 The clasping organ on lower male flagellum is slender and shaped as in S. cornicicluin. The 

 antennal squama tapers considerably towards the rather narrow, rounded end, and the tooth 

 on the outer angle is very small; squama reaches the middle of third antennular joint. Third 

 pair of maxillipeds are a little shorter than third pair of legs, but the three proximal joints 

 are broader than in these legs; the terminal joint (fig. id) is divided into 8 or 7 subjoints 

 (I have found both numbers in specimens from the same locality), but the interpretation is that 

 the joint is divided into 4 subjoints gradually distinctly shorter from the first to the fourth, and 

 that each of the three distal subjoints and sometimes besides the first subjoint is again sub- 

 divided into two joints. Fig. i b exhibits the long and thin terminal joint of first pair of legs. 

 The exopod of the uropods is about five times as long as broad and tapers from a little from 

 the base to the narrow end; scarcely two-thirds of its outer margin is ciliated, and there is no 

 marginal tooth. 



The branchiae are somewhat similar to those in S. arcticus\ first branchia above third 

 pair of legs is conspicuously more than twice as long as the second, which is about as long 

 as the second branchia above fourth legs, while the first branchia above fourth legs is some- 

 what or considerably longer, and more than half as long as first branchia above third legs. 



The petasma (figs, "ja — jc) shows interesting features. Lamina externa (Jam.) is distinctly 



