1 42 DISCOVERY REPORTS 



description of M. hanseni in some respects, principally in the small size of the eyes with the cornea 

 terminally situated and not extending laterally over the eyestalks, and also in the shape and proportions 

 of the rostral plate. There are, however, one or two outstanding differences. The peduncles of the 

 first and second antennae are of equal length in M. microps and I have not found any specimens among 

 the Discovery material in which this is not the case. In M. hanseni, on the other hand, the antennular 

 peduncle is much longer than the antennal peduncle which extends only to the distal margin of its 

 second segment. Zimmer stated definitely that the inner margin of the endopods of the uropods was 

 armed with setae only, whereas in M. microps there is an uneven row of fine spines among the setae. 

 Furthermore, though in both species the cleft of the telson is shallow and widely open, the armature 

 of the lateral margins of the telson is very different. 



In his second description of M. kerguelensis, Illig (1930, p. 472) mentioned that the male genital 

 organ was very long, and gave a figure in which it extended forward beyond the region of the mouth. 

 I have found specimens in the Discovery material in which the genital organ reaches far beyond the 

 distal end of the antennular peduncle, but in other characters the specimens conform so closely to 

 Illig's description that I have no hesitation in referring them to his species. The genital organ in- 

 creases with the growth of the animals in some species and may show great variation though in other 

 respects the males appear to be sexually mature, with dense hirsute brushes on the antennules. The 

 actual length of the organ is so variable that I do not consider that this character can safely be used for 

 the separation of species. 



Mysidetes posthon Holt and Tattersall, 1906 



1906a Mysidetes posthon Holt and Tattersall, p. 10. 

 1908 Mysidetes posthon, Tattersall, p. 33, pi. vn, figs. 

 1923 Mysidetes posthon, Tattersall, p. 287. 



Occurrence : 

 St. 39. 25. iii. 26 (day). Cumberland Bay, 235-179 m., over 50 adults, largest 15-4 mm., some $$ ovigerous, 10 juv. 

 St. 42. 1. iv. 26 (day). Cumberland Bay, 204-120 m., 1 c?> 12 mm., 2 adult $?, 16 mm.; fragments. 

 St. 45. 6. iv. 26 (day). Off South Georgia, 270-238 m., 14 adult $$, largest 15-6 mm.; 7 adult $$, largest 16 mm., 



few juv. 

 St. 123. 15. xii. 26 (day). Off South Georgia, 230-250 m., over 100, largest 17-2 mm. 

 St. 142. 30. xii. 26 (day). East Cumberland Bay, 88-273 m -> l acm lt 6*> 18-4 mm., 1 imm. S, 15 mm.; 5 imm. $$, 



15 mm.; many juv. up to 10 mm. 

 St. 143. 30. xii. 26 (day). Cumberland Bay, 273 m., 12 juv., largest 10 mm. 

 St. 144. 5. i. 27 (day). Cumberland Bay, 155-178 m., 3 juv., largest 11 mm. 

 St. 148. 9. i. 27 (day). Off South Georgia, 132-148 m., 3 S$, largest 14-4 mm.; 2 9$, 14 mm. 

 St. 152. 17. i. 27 (day). Off South Georgia, 245 m., 1 adult ?, 17-2 mm., 1 imm. $, 14-2 mm. 

 St. 154. 18. i. 27 (day). Off South Georgia, 60-160 m., 13 adult <$<$, 17-2-17 mm., 5 adult ?$, largest 17 mm., 



14 juv. 

 St. 156. 20. i. 27 (day). Off South Georgia, 200-236 m., 3 juv. 

 St. 167. 20. ii. 27 (day). Off Signy Island, South Orkneys, 244-344 m., 12 c?o*> largest 20 mm., 17 $$, largest 



22 mm. (those of 17 mm., quite imm.), 12 juv. 

 St. 181. 12. iii. 27 (day). Palmer Archipelago, 160-335 m -- 4 0*6* (largest 21 mm., adult), 6 $?, 18-22 mm.; none 



fully adult ; 3 juv. 

 St. 182. 14. iii. 27 (day). Palmer Archipelago, 278-500 m., 1 & 17 mm., 2 $?, 18 mm., all imm. 

 St. 187. 18. iii. 27 (day). Palmer Archipelago, 259 m., 1 adult $, 27 mm., with very small eighth thoracic endopod. 

 St. 190. 24. iii. 27 (day). Palmer Archipelago. Two hauls: (i) 93-126 m., 3 $<$, 19-21 mm.; (ii) 315 m., 1 adult <J, 



22 mm. 1 imm. $, 20 mm. 

 St. 363. 26. ii. 30 (day). South Sandwich Is., 329-278 m., 1 £, 18-5 mm., 2 fragments. 

 St. 368. 8. iii. 30 (day). South Sandwich Is., 653 m., 1 adult £, 22 mm. 



