1886.] DURING THE VOYAGE OF H. M.S. 'challenger.' 121 



the first four segments of the thorax are tuberculate, the two remam- 

 ing segments and the rudimentary terminal segment being smooth. 

 The abdominal appendages have epimera, which are directed down- 

 wards as in A. gigas. The operculiform first pair of thoracic 

 appendages are like those of the majority of species in consisting of 

 a single large basal joint, a small second joint, and a minute rudi- 

 mentary third joint. The remaining thoracic appendages are similar 

 to each other; the proximal joints are stout and bent, with short 

 stout spines. The abdominal appendages differ from those of the 

 last species in that both rami are setose, the outer perhaps more so 

 than the inner. 



Flinder's Passage, North Australia, 7 fathoms. 



The forty-four species which have been briefly described in the 

 foregoing pages do not include all the novelties in the ' Challenger ' 

 collection ; there are a few other species which I have not yet 

 examined with sufficient care to report upon, but which, so far as I 

 am aware at present, are undescribed forms. These include one 

 species of Arcturus, one of the allied genus Astacilln ; two species, 

 one from deep water, the other from Kerguelen, which are represen- 

 tatives of the genus Paranthura. The total number of new species 

 of Isopoda obtained during the voyage of the ' Challenger ' is 

 therefore about 70, comprising 10 new genera. 



Geographical and Bathymetrical Distribution. — It is interesting to 

 note that all the existing families of Isopoda without a single 

 exception ' are to be found in the deeper waters of the ocean ; but 

 the number of deep-sea species differs very considerably in the 

 diff'erent families ; the most characteristic are evidently the Mun- 

 nopsidse and Arcturidse, and, in a somewhat less degree, the Tanaids, 

 Asellids, and Serolidse ; the occurrence of a single species each of the 

 Anceidse, Cymothoidae, Anthuridse, and Sphseromidae may perhaps be 

 taken as an indication that these families are not largely represented 

 in the deep-sea fauna. 



In most cases the deep-sea species are distinct from the shallow- 

 water species ; only one or two (e. g. Arcturus fur catus, Studer) are 

 known, which are common to shallow water and the great depths. 

 In many cases the deep-sea genera are distinct, and this is particularly 

 so in the Asellids ; of this group, Acanthoniscus, Sars, Acanthomunna, 

 Trichopleon, lolanthe, are absolutely confined to deep water, while 

 Ischnosoma has four deep-sea species and only one which is an 

 inhabitant of shallow water in the extreme north, where the con- 

 ditions of temperature are much the same. Very frequently the 

 deep-sea Isopoda are distinguished by the extremely spiuy character 

 of the body ; this is largely the case with the deep-sea Arcturi, 

 and there are other instances. The great development of spines upon 

 the body is not, however, confined to the deep-sea Isopoda, but 

 is also found in many species from the colder regions, at Kerguelen, 



^ Sars has described a species of Idotheidaj, Glyptonottis meffalurus, which 

 enables me therefore to enunciate the general statement. 



