244 DISCOVERY REPORTS 



differences, so similar to one another, that there can be no doubt that the three species 

 are closely related. 



GROUP d 



The second natural group, Group d, comprises four species described in this report. 

 In addition to possessing spines on the third, fourth and fifth abdominal segments they 

 resemble one another and differ from all others in having, like Group a, a characteristic 

 secondary structure on the terminal process of the male copulatory organ. Half-way 

 along it in E. triacantha, much nearer the end in the other three, a short blunt strongly 

 chitinized secondary process arises from the inner or the front side;^ beyond it the 

 process is flattened and less heavily chitinized than before (Fig. 28). 



The order in which these species occur from north to south is as follows : 



E. hanseni: known only from the tropical zone off the west coast of Africa. 

 E. spinifera: circumpolar in the sub-tropical zone. 

 E. longirostris : circumpolar in the sub- Antarctic zone. 



E. triacantha : circumpolar in a narrow belt in the colder water of the sub-Antarctic, and through- 

 out the ice-free water of the Antarctic, zones. 



In some structural characters E. hanseni is nearer to E. spinifera than to either of the 

 others, E. spinifera stands midway between E. hanseni and E. longirostris, and the latter 

 midway between E. spinifera and E. triacantha. It is not easy to see these relationships 

 in all characters, but since they are shown by some, and since those which do not show 

 them do not point to any other order of relationship, I think it reasonable to believe that 

 the resemblances are the more significant, and that they show the true evolutionary 

 relationships of the species. 



The shape of the carapace is very similar in all, except that in E. triacantha it is not produced 

 anteriorly into post-ocular spines.- In E. hanseni there is a pair of strong sharp hepatic spines, each 

 arising from a low mound-like projection on the side of the carapace ; there are similar but blunter 

 spines in E. spinifera; in E. longirostris and E. triacantha there are no hepatic spines, but the low 

 mound-like projections remain, smaller in the latter than in the former (for these and the other com- 

 parisons in this and the next paragraph see Figs. 18, 19, 21 and 23). In E. hanseni the lateral denticle 

 of the carapace is set above the lower margin and behind a small angle in it; it is set above a deep 

 incision in the lower edge of the carapace in E. spinifera and E. longirostris; in E. triacantha it is on 

 the lower edge. The gastric area of the carapace is keeled in the mid-dorsal line in all; in E. spinifera 

 and E. lofigirostris the keel has a characteristic notch. 



The hinder edges of the terga of the third, fourth and fifth abdominal segments are indented 

 mid-laterally in E. hanseni; they are deeply incised in E. spinifera and E. longirostris, more so, in 

 that the third segment has more incisions, in the former than the latter; they are faintly indented 

 mid-laterally in E. triacantha. 



The characters of the antennular peduncles are shown in the following table (see Figs. 18, 19, 

 21 and 23). 



^ It is quite different in appearance from the slender thin-walled finger-like process near the end of the 

 terminal process in the seven species of Group a, and from the structure in a similar position in E. similis. 



2 Post-ocular spines are not found in any other species of the genus than E. hanseni, E. spinifera and 

 E. longirostris. 



