180 DISCOVERY REPORTS 



very convex distally, no proximal dilation, armed with a very dense row of small even teeth ; pair of 

 long plumose setae borne at base of cleft. There is a very conspicuous large chromatophore in the 

 median line of the telson about two-thirds of its length from the base (Fig. 46 P). 



Length. Largest female, 31 mm.; largest male, 30 mm. 



Distribution. All the records of this species are from the Strait of Magellan with the exception of 

 station WS 749, half-way between Tierra del Fuego and the Falkland Islands. 



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hip,. 45. Arthromysis magellanica (Cunningham). A, anterior end of adult female in dorsal view, xa; B, adult female 

 in lateral view, x 6; C, labrum; D, eighth thoracic appendage of adult male, x 14. 



Remarks. In all essential characters this species conforms to the definition laid down by Hansen 

 for the tribe Mysini, but it possesses one character, which has not before been recorded for this 

 tribe — the presence of long plumose setae at the base of the cleft of the telson. These setae are 

 characteristic of the tribe Erythropini and occur in one or two genera of the tribe Leptomysini. Since 

 all the other characters of the genus Arthromysis are definitely those of the tribe Mysini, the presence 

 of these conspicuous setae, in conjunction with the very large number of sub-segments in the tarsus 

 of the thoracic endopods, simplifies the identification of both sexes of the species. The only species 

 with which the females might be confused is Tenagomysis tenuipes, in which there may be as many 

 as 14 sub-segments in the tarsus of the third to seventh thoracic endopods and in which the presence 

 of plumose setae arming the base of the cleft of the telson is a generic character. A. magellanica may 

 be distinguished from this species by the very long, slender eyes and by the semicircular shape of 

 the anterior margin of the carapace. 



