60 DISCOVERY REPORTS 



material. In the Discovery specimen the form of the antennule agrees precisely with that found in 

 males of H. fyllae, the proximal 16-18 segments of the outer flagellum being enormously enlarged, 

 especially proximally (Fig. 8 A). The antennal scale agrees closely with that of Tattersall's specimens, 

 but the antennal peduncle differs in that the third segment is larger than the second and in his 

 description the reverse is the case (Fig. 8 A, C). The thoracic appendages and, as far as can be ascer- 

 tained without dissection, the mouth parts are similar to Tattersall's description. The endopods and 

 exopods of the uropods are sub-equal in length. The outer margin of the proximal segment of the 

 exopod bears no setae, but is armed along the distal two-thirds of its length with a row of 25-26 evenly 

 graduated spines which increase in size distally (Fig. 8 F). The portion of the telson which is present 

 differs in shape from Tattersall's figure, but he himself pointed out that his representation of this organ 

 was largely conjectural (Fig. 8 F). 



The chief difference shown by this specimen is in the form and armature of the carapace and of the 

 posterior somites of the thorax. The carapace is extremely short, leaving the whole of the last three 

 thoracic somites completely exposed. The anterior end is evenly arcuate with distinctly upturned rim. 

 The antero-lateral angles, which Tattersall describes as ' evenly rounded ', are produced into acute 

 angles which extend forward beyond the anterior margin of the rostrum. The most remarkable feature 

 of this specimen is the moulding of the surface of the carapace. On each flank it is thickened and rises 

 into two distinct longitudinal keels. Posterior to the cervical sulcus, there is an oval, level, depressed 

 region, marking the area of attachment of the carapace with the thoracic somites, bounded by a curved 

 keel on each side. Half-way along this keel, there is a sharp spine. The keel is marked at its anterior 

 end by a second spine and then turns and runs transversely downward toward the lateral margin of the 

 carapace. A second keel runs longitudinally, close to the posterior half of the lateral margin. In 

 addition a number of spines adorn the carapace as shown in Fig. 8 A, B. 



The large hepatic spines are particularly robust and appear to be freely articulated in a kind of 

 ball-and-socket joint (Fig. 8 A, B). The dorsal anterior margin of the sixth thoracic somite is produced 

 forward into a strong membranous flap, which appears in lateral view as a robust blunt process. The 

 terga of the seventh and eighth somites are enlarged and thickened and override the posterior margins 

 of the preceding somites (Fig. 8 B). The first abdominal somite is larger and longer than the others and 

 thickened on its dorsal surface to form a saddle similar to that described by Tattersall. The remaining 

 abdominal somites taper considerably towards the telson. 



The spines (Fig. 8 A, B) on the carapace and the four very strong, median denticles are so con- 

 spicuous that it is surprising that other workers mention only a median tooth with a smaller one 

 posterior to it. It may well be that, as all previous specimens were mutilated, some of the spines had 

 been broken off, but it is difficult to understand why neither Tattersall nor Hansen commented upon 

 the strong keels, which are so conspicuous a feature of the Discovery specimen. 



Hansen (1913, p. 8), who described the first male seen, stated that the eyes were reduced to two 

 separate, extremely small triangular lappets barely extending beyond the anterior margin of the 

 rostrum. He suggested that the differences between them and the type specimens might be sexual. 

 In the Discovery specimen, which is also an adult male, the eyes are almost precisely as described and 

 figured by Tattersall for his females and it seems probable that the eyes of Hansen's male had been 

 partially torn away. 



The male pleopods of this specimen have not previously been figured, but Hansen (1913, p. 8) 

 recorded that they are similar to those of H. fyllae, except that in the fifth pair the endopod is 

 considerably longer than the exopod (Fig. 8E). The first pair of pleopods in the Discovery specimen 

 appears to differ from the other species in the genus in having the endopod two-segmented, but I 

 have not dissected it and it may be that the articulation is incomplete (Fig. 8D). 



