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Show 1886.] DURING THE VOYAGE OF H.M.S. 'CHALLENGER.' 121 the first four segments of the thorax are tuberculate, the two remaining segments and the rudimentary terminal segment being smooth. The abdominal appendages have epimera, which are directed downwards 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 rudimentary 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 Astacilla ; two species, one from deep water, the other from Kerguelen, which are representatives 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 andBathymetrical Distribution.-It is interesting to note that all the existing families of Isopoda without a single exception1 are to be found in the deeper waters of the ocean ; but the number of deep-sea species differs very considerably in the different families ; the most characteristic are evidently the Mun-nopsidse and Arcturidae, and, in a somewhat less degree, the Tanaids, Asellids, and Serolidae ; the occurrence of a single species each of the Anceida?, Cymothoidae, Anthuridae, 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, Iolanthe, 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 conditions of temperature are much the same. Very frequently the deep-sea Isopoda are distinguished by the extremely spiny 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, 1 Sars has described a species of Idotheidse, Glyptonotus megalurus, enables me therefore to enunciate the general statement. |