y.. Third joint of the antennular peduncles as long as or longer than first joint. 



Species i — 2 : S. atlanticus M.-Edw. [S. Frisii Kr.), and 

 5". cornutus Kr. 



(3. Third joint of the antennular peduncles very considerably shorter than first joint. 



Spec. 3 — 4: S. arcticus Kr., and 5. affinis H. J. H. 



b. The two distal joints of the antennular peduncles considerably or much thickened ; 

 third joint slightly or very considerably shorter than the inner margin of the second. 

 Supra-ocular spines always, hepatic spines most frequently wanting. 



a. Eyes seen from above scarcely or at most a little broader than their stalks. 

 Third joint of the antennular peduncles slightly shorter than the inner margin of 

 second joint. Integument of the body soft and completely membranaceous. 



Spec. 5 — 7: S. japonicus Bate [S. mollis Smith), S. inozes 

 Faxon, and S. profundus Bate. 



r {j. Eyes seen from above considerably or much broader than their stalks. Third 

 joint of the antennular peduncles somewhat or considerably shorter than the inner 

 margin of second joint. Integument of the body moderately soft or somewhat 

 rigid, less membranaceous. 



f. No luminous organs on the body or the appendages. 



§. A process or protuberance on the inner side of the eye-stalks near 

 the eye. Spec. 8 — 9: S. Kröyeri Bate, and S. Gardineri Kemp. 



§§. No distinct protuberance on the eye-stalks (in 5. phorcus a feeble 

 rudiment of a protuberance). 



Spec. 10 — 13: 6\ phorcus Faxon, 5. bisulcatus Wood- 

 Mas., 5. prehensilis Bate, and 6". robushis Smith. 



ff. Numerous luminous organs distributed on the body, especially on its lower 

 surface, and on most of the appendages. 



Spec. 14 — 17: S. Challengeri H. J. H., 6". fulgcns n.sp., 

 (S. splcndcns n. sp. in MS., Atlantic), and 5. glorio- 

 sus Stebb. 



B. On the exopod of the uropods the ciliated part occupies somewhat more than half of the 

 exterior margin. (Slender forms with the antennular peduncles long). 



Spec. 18 — 19 : 5". cornicuhim Kr., and S. seminudus n. sp. 



It may be added that this conspectus is not only an analytical key aiding at the deter- 

 mination of specimens, but it gives besides a kind of classification, as the forms are collected 

 into small natural tribes. {Acheles arachnipodus Cocco (1832), which seems to be a species of 

 Sergestcs, is omitted, as it is probably impossible to recognize the form with complete certainty; 

 "Sergestes arachnipodus De Natale (ex Cocco)" as described and figured by Riggio (1905), must, 

 according to the equipment with spines of processus ventralis of the petasma, be S. arctictis Kr.). 



The material collected by the "Siboga" comprises adult specimens of seven species. 



