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



because in many cases, particularly among the Sphaeromids, carcinologists arc by no 

 means agreed as to the limits of particular genera. The following genera, however, among 

 others, appear to be, as far as is at present known, peculiar to the littoral zone : — 



Asellus. 



Jasra. 



Paramnnna. 



Dendrotion. 



Idothea, and other allied genera. 



Anthura. 



Haliophasma. 



Sphaeroma, and allied genera. 



Ceratocephalus. 



Many genera of Cymothoidae. 



At any rate it may be safely stated that there are more genera peculiar to the littoral 

 zone than either to the continental or abyssal ; the continental zone is evidently a tran- 

 sitional area combining to a certain extent the characters of the abyssal and littoral zones, 

 its own special peculiarities being but feebly marked. Seeing, however, that out of the 

 thirty genera assigned to this region, twenty-five are also found in shallow water, and 

 only fourteen in deeper water (thirteen of these being genera which are also found in 

 shallow water), it appears to me to be almost unnecessary to regard this transition.il 

 zone in considering the bathymetrical distribution of the Isopoda. The older division of 

 the marine fauna into two sections, the deep-sea fauna and the shallow-water fauna, fixing 

 the limit between them at about 300 fathoms, appears to me to meet the requirements 

 of the case. All the abyssal genera which are found in shallow water at all are 

 met with in depths of less than 100 fathoms, with the sole exception of Nannoniscus, 

 which, being only represented by a single species, cannot be considered to form an 

 important exception. Looking at the matter from this point of view, we shall have eleven 

 peculiar genera of the abyssal fauna, and only four species out of sixty-five which are 

 common to this zone and to the shallower waters. 



Mr. John Murray and M. Eenard have lately expressed the opinion that " dredgings 

 near the shores of continents in depths of 1000, 2000 or 3000 fathoms are more productive, 

 both in species and individuals, than dredgings at simdar depths several hundred miles 

 seaward"; and also that " among the few species dredged in the abyssal areas furthest 

 removed from land, the majority show archaic characters, or belong to groups which have 

 a wide distribution in time as well as over the floor of the present oceans." It will be 

 interesting to see how far these conclusions, deduced from a study of other deep-sea 

 invertebrata, are applicable to the Isopoda. With regard to the first question, I find on 

 reference to the list of deep-water forms of Isopoda that there are twenty-five that were 

 dredged in the neighbourhood of continental land, and twenty-two at a long distance 

 from any continental land, though most frequently in the vicinity of land, e.g., at 

 Kerguelen, whde four species are found in both habitats. As regards number of species, 

 therefore, the distribution of the deep-sea Isopoda does not lend any strong support to 



