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



the middle, between the two larger apical spines, projects moreover a thin and finely- 

 serrate lamella, to the lower side of which are attached two delicate diverging bristles. 

 This serrate lamella is not figured distinctly in the work of the late Professor M. Sars, 

 although it is mentioned in the text. However, on re-examining the Norwegian form 

 I have found it to be present, and in form and armature of precisely the same appearance 

 as in the specimen here figured (fig. 7). 



Habitat. — The specimens procured by the Challenger Expedition were collected at 

 the followino- Stations : — 



Station 141, December 17, 1873 ; lat. 34° 41' S., long. 18° 36' E.; depth, 98 fathoms; 

 green sand ; bottom temperature, 49°'5. 



Station 142, December 18, 1873; lat. 35° 4' S., long. 18°37'E.; depth, 150 fathoms; 

 green sand; bottom temperature, 47°'0. 



The present species occurs rather abundantly along the southern and western coasts 

 of Norway at a depth of from 20 to 100 fathoms, and has also been recorded from the 

 Shetland Isles by the Rev. Dr. Norman ( = Ctenomysis alata of that author). 



It may be regarded as a true bottom-form, never having been found at the surface of 

 the sea, as is the case with some other Schizopods. 



Distnhution. — Concerning the geographical distribution of the species, the occurrence 

 of this form in the southern hemisphere, as shown by the Challenger collection, is remarkable, 

 and might induce the belief that it ranges from the Norwegian Sea along the whole western 

 coast of Europe and Africa, or thi-oughout the boreal, lusitauic, tropic, and antiboreal 

 regions. It may, however, be considered as a highly remarkable fact, that this very 

 striking form has never been recorded either from the coasts of England and France, or 

 from the Mediterranean, although each of these tracts has been carefully investigated by 

 numerous zoologists. We may therefore entertain the assumption that this form in 

 reality does not occur throughout the intermediate tracts of the ocean, but is met with 

 independently in both hemispheres in the corresponding region. Should this be the case, 

 we may infer that the distribution of the species must at an earlier date have been 

 continuous, but considerable changes afterwards occurring in the physical conditions led 

 to a separation of the species into two independent stocks. In the sequel we shall meet 

 with another still more striking example of a similar kind, in treating of the Mysidian 

 Boreomysis scypliops, a form stated to occur in the Arctic and Subantarctic regions only, 

 having never yet been found in any intervening tract. 



