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



would not seem to be fibrous but rather of a similar cellular structure to that of the 

 wantrlia themselves. On the other hand, the ganglia of the tail are connected by very 

 lono- and narrow, distinctly fibrous commissures, placed close together. 



The colour of the animal, in a fresh state, as observed on the Norwegian North 

 Atlantic Expedition, was a uniform pale red. 



Habitat. — Several specimens of this magnificent Mysidan were collected on the 

 Challenger Expedition in the Southern Ocean from very great depths. The following is 

 a list of the Stations : — 



Station 147, December 30, 1873 ; lat. 46° 16' S., long. 48° 27' E. (off Crozet Islands) : 

 depth, 1600 fothoms ; Diatom ooze; bottom temperature, 34°"2. 



Station 157, March 3, 1874 ; lat. 53° 55' S., long. 108° 35' K; depth, 1950 fathoms; 

 Diatom ooze ; bottom temperature, 32° "l. 



Station 158, March 7, 1874 ; lat. 50° 1' S., long. 123° 4' K; depth, 1800 fathoms; 

 Globigerina ooze ; bottom temperature, 33°"5. 



Distribution. — The geographical distribution of this form is very remarkable, ranging, 

 as it does, within the limits of well nigh the same region in both hemispheres, and not 

 occurring in the intermediate tracts of the ocean. As it cannot be reasonably assumed 

 that the species has originated independently in both hemispheres, the physical condition 

 of the sea-bottom must at some time or other have been more uniform than at present, 

 to have admitted of the species spreading over a much more extensive area, whereas at a 

 later period essential changes in the climatological conditions must be assumed to have 

 caused this form to withdraw successively from the equatorial region towards the two 

 poles, thus dividing the species into two widely separated stocks, inhabiting correspond- 

 ing regions in both hemispheres. Another fact, too, viz., the remarkable occurrence, as 

 stated above, of the northern form, Lophogaster typicus, M. Sars, in the Southern Ocean, 

 without its being ever met with in the intermediate tracts, may also warrant the assump- 

 tion of essential changes in the physical conditions of the sea-bed having taken place at 

 some former period, thus causing the occui-rence of certain species to appear discontinuous. 



44. Boreomysis obtusata, G. 0. Sars (PL XXXIII. figs. 1-6). 



Boreomyds obttisata, G. O. S;iis, Preliminary Notices on the Challenger Schizopoda, No. 42. 



Specijic CJmracters. — Male : — Anterior part of carapace not keeled above nor forming 

 anj" distinct rostral projection, the frontal margin being bx'oadly rounded, with but a very 

 slight angle in the middle ; antero-lateral lobes sharj)ly pointed. Eyes well developed, 

 rather short and broad, almost circular, cornea greatly expanded, with reddish pigment. 

 Antennular peduncle rather stout, with second joint very short, discoidal. Antenna! 

 scale oblong, tapering, upex narrowly truncate, denticle of outer corner but slightly 

 projecting. Propodal joint of legs subdivided into three distinctly defined articulations. 



