218 "TERRA NOVA" EXPEDITION. 



Milne-Edwards' original description says nothing about the length of the process 

 from the seventh thoracic somite. He merely stated, " Septieme segment du thorax 

 surmonte d'une dent conique mediane dirigee en arriere." 



Dana, in 1853, described two species from the Bay of Islands, one of which he 

 referred to S. armata, M.-Ed., with " seventh thoracic segpent having a tooth behind, 

 the tooth sometimes obsolescent " ; and the other, which he describes as a new species, 

 S. spinigera, having the " tooth of the seventh thoracic segment elongate, spiniform, 

 longer than half the abdomen." Dana also states that in his S. armata the caudal 

 lamellae do not quite reach the apex of the telson, whereas in S. spinigera they extend 

 beyond it. I am of the opinion that Dana's S. armuta, with the seventh thoracic 

 somite having a tooth behind, is the young male, and those specimens with the tooth 

 obsolescent adult females ; and his S. spinigera, with the elongate spiniform tooth on 

 the seventh thoracic somite, the fully adult male of the same species, which I regard, as 

 the same as Milne-Edwards' species. 



Miers founded the genus Isocladus on a number of specimens which included both 

 females and adult males, and which he referred to Dana's S. sjnnigera (?). He had not 

 seen any specimens which he could refer to S. armata, M.-Ed., and if any young males 

 occurred among the specimens which he examined he probably regarded them as 

 merely growth stages of /. sjmiigera, without considering whether they agreed with 

 Milne-Edwards' description of S. armata. 



There seems to be some confusion as to the identity of this species in the minds of 

 the New Zealand zoologists who have collected specimens. Thomson and Chilton (1883) 

 record I. armatus from the Bay of Islands on the authority of Thomson, and /. spiniger 

 from Lyttelton on the authority of Chilton ; Thomson adding a note to the latter 

 record, " I do not think I know this form." 



Thomson (1913) records both species from Otago Harbour, but seems doubtful of 

 his record of /. spiniger, since he adds, " Several specimens collected near Dunedin 

 appear to belong to this species." 



The figures here given will show the general form of the body in the adult and 

 young male and in the adult female. The body is strongly convex, and capable of 

 being rolled up into a ball. It is smooth and without tubercles of any kind. The last 

 segment of the abdomen is strongly convex in the centre, with a flatter marginal 

 portion. There is a median shallow groove or depression in the central convex portion, 

 which gives the impression that the latter is made up of two obscure bosses. The 

 spiniform process from the seventh thoracic somite of the male reaches backward nearly 

 to the apex of the telson. In the young male it appears as a short tooth, and is absent 

 in the female. 



The uropods in the adult male extend slightly beyond the apex of the telson ; 

 inner ramus broad and ovate, with a truncate tip ; outer ramus scythe-like, curving 

 slightly outwards ; apex pointed. In the female the uropods are smaller than in the 

 male, and do not reach the apex of the telson, but are otherwise of the same general 



