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



being furnished as in that species with numerous slender spines inserted on tubercular 

 projections of the limb. 



The uropoda consist of a single short conical joint. 



This species is evidently more nearly allied to Ischnosmnn spinosum than to either of 

 the two species, Ischnosoma quadrispinoswm and Ischnosoma Mspinosum, described by 

 Sars. It agrees with the former species in the great devel< •] iment of spines upon the lateral 

 regions of the thoracic segments. The specimen described here is a male, and it is 

 therefore possible that it is really the male of Ischnosoma spinosum. So little is a1 

 present known with respect to this interesting genus, all the species having been described 

 from single specimens, that it is impossible to settle this point. For the present I shall 

 regard this species as distinct. 



The points in which it differs from Ischnosoma spinosum are very numerous. In the 

 first place the fifth segment of the thorax is relatively as well as actually longer than in 

 that species. The lateral spines of the thoracic segments are much longer, as well as more 

 slender, and sharper at their extremities in Ischnosoma spinosum than in Ischnosoma 

 thomsoni ; similarly the spines upon the thoracic appendages are better developed in the 

 former than in the latter species; on the other hand, the fourth segment has a row of 

 lateral spines in Ischnosoma thomsoni which are absent in Ischnosoma spinosum. The 

 head is considerably smaller in proportion in Ischnosoma spinosum, and the tuft of 

 short spines placed anteriorly to the antennules, which are characteristic of that species, 

 are wanting in Ischnosoma thomsoni. 



The most salient difference between the two species is, however, in the form of the 

 abdominal shield and the uropoda. 



In Ischnosoma spinosum the posterior extremity of the caudal shield is truncated, 

 and the uropoda, which are curved and cylindrical in form, with a pointed extremity, 

 project beyond it. 



In Ischnosoma thomsoni the abdomen ends in a prominent bifurcate extremity, and the 

 uropoda, which are more flattened, do not reach nearly as far as the extremity. 



In the former species there are a pair of very short spines (omitted in my previous 

 description, but illustrated in the figure) at the end of the aMomen, but these are similar 

 to the spines upon the thoracic limbs, being clear and transparent, and fixed on a short 

 projection of the calcareous integument. 



The occurrence of this species at Station 246 extends the range of the genus into the 

 North Pacific; the depth is greater than that which the genus has been previously known 

 to inhabit. 



Station 246, July 2, 1875; lat. 36° 10' N., long. 178° 0' E.; depth, 2050 fathoms; 

 bottom temperature, 35° - l F.; bottom, Globigerina ooze. 



