200 CRUSTACEA CHAP. 
In the Antarctic zone we are presented with very different 
relations, since the great continents are drawn out to points 
towards the south, and are isolated by vast tracts of imtervening 
deep sea. Nevertheless, certain littoral forms are circumpolar, 
e.g. the Palinurid Zasus and the Crabs Cyclograpsus and Hymeno- 
soma. The genus Dromidia is common to Australia and South 
Africa, though it is apparently absent from South America. 
The Isopod genus Serolis is confined to Antarctic seas. The 
majority are littoral species, and they are distributed round the 
coasts of Patagonia, Australia, and Kerguelen in a manner that 
certainly suggests a closer connection between these shores in the 
past. These facts are, on the whole, evidence in favour of the 
former existence of an Antarctic continent stretching farther 
north and connecting Australia, Africa, and 8S. America—a 
supposition that has been put forward to account for the dis- 
tribution of the Penguins, Struthious birds, Oligochaets, Crayfishes, 
etc., in these regions (see pp. 215-217). 
In considering the Arctic and Antarctic faunas the supposed 
phenomenon of bipolarity must be mentioned, z.e. the occurrence 
of particular species in Arctic and Antarctic seas, but not in the 
intermediate regions. This discontinuous type of distribution 
was upheld for a variety of marine animals by Pfeffer, Murray, 
and others, but it has been very adversely criticised by 
Ortmann.’ As far as the Arctic and Antarctic Decapod fauna 
in general are concerned, the north polar forms are quite distinct 
from the south polar. Typical of the former are Hippolyte, 
Sclerocrangon, Hyas, Homarus, etc.; of the latter, Zymenosoma, 
Dromidia, Iasus. It appears, however, that in certain special 
cases, bipolarity of distribution may be produced owing to the 
operation of peculiar causes. Two such cases seem to be fairly 
well established. Crangon antarcticus occurs at the two poles, 
and apparently not in the intermediate regions; but, as Ort- 
mann points out, it is represented right down the West American 
coast by a very closely related form, C. franciscorum. The 
waters on the tropical western coasts both of Africa and America 
are exceedingly cool, and it appears that in this way the Crangon 
may have migrated across the tropical belt, leaving a slightly 
modified race to represent it in this intermediate region. The 
other case of bipolarity is afforded by the “Schizopod,” Loreo- 
1 American Naturalist, xxxiil., 1899, p. 583. 
