SYSTEMATIC REPORT i79 



This genus agrees with the genus Antarctomysis in many of its characters, but may be distinguished 

 from it by the simple unsegmented endopods of the third and fourth male pleopods ; by the very long 

 slender eyes ; by the narrow second segment of the mandibular palp ; by the very large number of sub- 

 segments of the carpo-propodus of the third to the seventh thoracic endopods and by the presence of 

 a pair of very long plumose setae at the base of the cleft of the telson. 



Arthromysis magellanica (Cunningham), 1871 



(Fig.45A-D; 4 6A-P) 



1 87 1 Macromysis magellanica Cunningham, p. 497. 

 19156 Antarctomysis sp. Zimmer, p. 170. 

 1924 Arthromysis chierchiae Colosi, p. 3, figs. 



Occurrence : 



St. WS 89. 7. iv. 27 (day). Midway between the Falkland Is. and Tierra del Fuego, 23-21 m., 64 SS, 59 ??» 



28-31 mm., many breeding. 

 St. WS 749. 18. ix. 31 (day). Strait of Magellan, 4o(-o) m., 21 adult 33, 14 ?$ (5 ovig.), 22-31 mm., 9 juv. <$$, 



fragments. 



• Description. General form long and slender. Carapace short posteriorly, leaving the whole of the 

 last two and part of the sixth thoracic somites exposed in dorsal view; anterior margin semicircular 

 with, in a few large specimens only, a hint of an angle in the median line ; antero-lateral angles produced 

 into very acute spine-like processes (Fig. 45 A, B); carapace very shallow laterally with the margin 

 anterior to the cervical sulcus sinuous (Fig. 45 B) ; posterior margin transverse with only a very slight 

 emargination in the median region; postero-lateral angles rounded. Abdomen strong and muscular; 

 anterior five somites sub-equal; sixth nearly twice as long as the fifth (Fig. 45 B). Antennular peduncle 

 long and slender especially in the female; second segment the shortest, third segment expanded at its 

 distal end (Figs. 45 A and 46 A); scale long and slender, more than twice as long as the antennular 

 peduncle; small distal suture present (Figs. 45 A and 46 B). Mandibular palp slender, with second seg- 

 ment not expanded (Fig. 46 C). Maxillule, maxilla, first and second thoracic appendages as shown in 

 Fig. 46D-G. Third to the eighth thoracic appendages with the endopods not particularly long; carpo- 

 propodus with 24-26 sub-segments in the third to seventh endopods and with ten in the eighth. Male 

 genital organ very large, barrel-shaped with a well-marked lobe at its distal end (Fig. 45 D). Marsu- 

 pium composed of three pairs of brood lamellae, those on the sixth thoracic appendages very small. 

 Pleopods of the female rudimentary, in the form of unsegmented plates, which become progressively 

 longer on the posterior somites (Fig. 45 B). Pleopods of the male ; first and second pairs as in the female 

 (Fig. 46 H, J); third pair with unsegmented endopod (Fig. 46K); fourth pair with simple unsegmented 

 endopod ; exopod extremely long, extending backward beyond the apex of the telson, composed of ten 

 segments marked off by very obscure articulations; penultimate segment very short, armed with one 

 strong, long seta, which is armed along its outer margin with a regular row of small spines ; distal 

 segment long and slender, armed distally with two unequal spinous setae and a simple seta (Fig. 

 46 L) ; fifth pair large, normal, with the exopod slightly longer than the endopod (Fig. 46 M). Uropods, 

 very long and slender, exopod more than half as long again as the endopod, which is armed along 

 the inner margin with 17-19 spines among the setae (Fig. 46 N). Telson broad and oblong, lateral 

 margins nearly parallel, armed throughout their whole length with 25-26 evenly spaced spines ; distal 

 spine long and placed very close to the small apical spine which is less than half its length (in some 

 specimens this long spine appears to be borne on the apex but in the majority of the animals it is 

 definitely situated on the lateral margin); cleft about one-sixth of the length of the telson, its margins 



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