84 THE VOYAGE OF H.M.S. CHALLENGER. 



The three posterior thoracic appendages are modified in the swimming feet ; the basal 

 joint is comparatively long and stout ; the two succeeding joints dei-nase progressively 

 in length; the next joint is long, flattened, and crescent-shaped, the outer margin being 

 thickened; it is fringed on both sides with long plumose hairs ; the fifth joint is rather 

 narrower, and apparently not so much compressed and flattened ; it is fringed only 

 on the inner side with similar plumose hairs; the distal joint is long and narrow; it 

 terminates in a few longish hairs, and its outer side is closely fringed with very short 

 hairs. The same remarks about the similarity of these appendages to the last thoracic 

 limbs of Ilyarachna may be made on this species as on the last. 



The uropoda (fig. 3) are long and styliform, reaching nearly to the extremity of the 

 telson spine; they appear to be composed of three separate joints, the first and third 

 being subecpial and longer than the middle one. 



It is evident from the foregoing description that this species presents many points of 

 difference from Acanthocopc spinicauda; since, however, both species are only represented 

 by a single individual, which happen to be of opposite sexes, it is impossible to say with 

 certainty whether some of the points of difference may not be sexual rather than specific ; 

 in the mean time, however, there is no other Munnopsis known in which the two sexes 

 differ in so many points and in such points, though the fact that the present species are 

 representatives of a new genus must be borne in mind in considering the question. 



Eegarding the two individuals as representing two distinct species, the points of 

 difference are shortly as follows. 



In Acanthocopc spinicauda the three posterior thoracic segments taken together are 

 about twice the length of the four anterior. 



In Acanthocopc acutispina the three posterior thoracic segments are shorter than the 

 four anterior. The latter species also differs from the former in the absence of any long 

 tergal plume and in the much greater length of the epimeral spines. 



The two species also appear to differ in the relative development of the short spines 

 which beset the general body surface; in Acanthocope spinicauda these spines are 

 certainly visible on some of the epimeral spines, but they seem to be confined to the 

 posterior lateral margin of the epimeron. 



The most marked point of difference between the two species is in the form of the 

 abdominal shield and of the uropoda. 



In figs. 2 and 12 of PL VIII. are drawings of the abdominal shield in the t wo species ; 

 in Acanthocopjc spinicauda (fig. 12) the telson is very much longer than it is in 

 Acanthocope acutispina, but at the same time the lateral spines arc rather shorter in pro- 

 portion to the length of the abdominal shield than in the last named species. The antennules 

 are also remarkably different; the shortness of the flagellum of Acanthocopc noitispina 

 contrasts with the very long flagellum of Acanthocopc spinivanda. The uropoda of 

 Acanthocope acutispina, have only three joints ; those of Acanthocope spinicauda five. 



