ig2 DISCOVERY REPORTS 



cormitus, on the other hand, came from Marion Island. I have examined one of his identified 

 specimens in the British Museum (Natural History). The specimen is a male and possesses a pointed 

 pleotelson as in acuminatiis and therefore differs from Studer's genotype, cormitus. Beddard's 

 specimen is slightly larger than the male in the Discovery collections and lacks body spines, except 

 for a pair on the head; the spines on the male specimen in the Discovery collections are less well 

 developed than those of the female. Both Discovery specimens were taken in the neighbourhood of 

 Prince Edward Island, that is, in the same area as Beddard's specimen. 



Text-fig. 1 8. Arcturides acuminatiis sp.n. (a) First pleopod (right) S, x 34- (^') Ptnis, x 34. 

 {c) Second pleopod ^ with exopodite removed, x 34. 



Prince Edward and Marion Islands are separated by about 1 800 miles of deep water from Kerguelen 

 Island; this must almost certainly act as a barrier to the spread of a shallow- water species from one 

 locality to the other. I consider that the specimens collected from these two areas, although very 

 similar, represent two distinct species, and that Beddard's material, recorded as A. cormitus, more 

 properly belongs to the new species just described, A. acuminatiis. 



Xenarcturidae, fam.n. 



Diagnosis. The body is dorso-ventrally flattened, without any elongation of the fifth thoracic 

 somite; the second thoracic somite has its middle portion completely fused with the head, but its 

 lateral pleural extensions are free. The tergum of each of the second to the fifth thoracic somites 

 inclusive (first to fourth pereion somites) has on either side a pleural extension which extends 

 laterally over the base of the limb. On the ventral surface of these somites each coxal joint is clearly 

 defined; its outer margin is ring-like and, except for those of the second somite, is separated from the 

 ventral surface of its pleuron by a suture. The inner margins of the coxal joints of these limbs are 

 expanded into coxal plates ; each extends over the ventral surface of its somite almost to the mid-line. 



