40 THE VOYAGE OF H.M.S. CHALLENGER. 



tives of the genus. Ischnosoma bacillus is characterised by the presence of a long lateral 



.■spine on the fifth as well as on the fourth segment of the thorax. Ischnosoma bacilloides 

 may be distinguished from its congeners, by the fact that both the sixth and fifth 

 segments of the thorax have the lateral spines. Other differences will be pointed out in 

 the course of the following descriptions of these species. The third species, which I have 

 named Ischnosoma spinosum, is a near ally of Sara's two species, and differs very consider- 

 ably from Ischnosoma bacillus and Ischnosoma bacilloides. The genus as at present 

 known falls into two subdivisions ; the northern species Ischnosoma spinosum, Ischnosoma 

 bispinosum, and Ischnosoma quadrispinosum are distinguished by their small size, by the 

 comparatively small size of the fourth and fifth segments of the thorax as compared with 

 the hinder end of the body, and by the great length of the first free abdominal segment. 

 The antarctic species, Ischnosoma bacillus and Ischnosoma bacilloides, are characterised 

 by their large size, by the great length of the fourth and fifth segments of the thorax, and 

 by the extreme shortness of the free abdominal segment. 



Ischnosoma spinosum approaches the genus Munnopsis more nearly than any of Sara's 

 species by reason of the fourth thoracic segment, which is much more like the preceding, 

 and is apparently not so elongated as in Ischnosoma bispinosum and Ischnosoma quadri- 

 spinosum. In this species, therefore, as in Munnopsis, the thorax consists of two distinct 

 regions, an anterior under portion including the first four segments, and a posterior 

 narrow portion consisting of the three posterior segments of the thorax. 



Ischnosoma spinosum, F. E. Beddard (PL VI. figs. 1-5). 



Ichnosoma spinosum, F. E. Beddard, Proc. Zool. Soc. Loud., 188G, pt. i. p. 99. 



The present species is represented by a single individual which is complete, with the 

 exception of one or two of the thoracic limbs. It resembles Sara's two species, Ischnosoma 

 quadrispinosv/m and Ichnosoma bispinosum, much more closely than do either of the 

 other species described here. 



The specimen measures about 6 mm. in length and is a female, with short immature 

 ovigerous lamellse upon the first four thoracic segments. 



The anterior region of the body, comprising the head and first four segments of the 

 thorax, is broader than the posterior region but not so long ; it equals in length the three 

 posterior thoracic segments, plus the single free abdominal segment ; the general shape of 

 the body is in fact precisely similar to that of Ischnosoma quadrispinosum. 1 



The head is very much narrower and shorter than the first segment of the thorax; the 

 frontal margin is rounded, and at either side, in front of the articulation of the antennules, 

 there is a short projecting lobe fringed with spines; behind this there is a deep semi- 

 circular notch where the basal joint of the antennules is fixed ; there is no trace of any eyes 



1 G. 0. Sars, Norske Nortlhavs Exped., &c, pi. xi. fig. 26. 



