474 Proceedings of Royal Society of Edinburgh. [sess. 
and the third being referable to Willem’s recently described 
Antarctic genus Cryptopygus. The two species of Isotoma 
indicate, as will be seen, affinities of the apterous fauna of the 
South Orkneys to that of other Antarctic lands eastwards and 
westwards, as well as to that of the Arctic regions, and even to 
that of the land whence the Scotia sailed. 
Two of the species are so abundantly represented that a study 
of the jaws has been possible. In this research much help has 
been gathered from the recent careful paper by Folsom (1899) 
on the mouth-parts of Orchesella. 
Description of Species. 
Isotoma Brucei ) sp. nov. PI. I. figs. 1-8. 
Length 1*75 mm. Feelers as long as head. Six ocelli on each 
side of head (fig. 2). Ridge surrounding the post-antennal organ 
elongate and narrow (fig. 2, p. a. o.). Feet without tenent hairs, 
claws slender and untoothed (fig. 5). Spring evidently borne on 
the fourth abdominal segment, slender and elongate, one-fourth 
length of insect ; manubrium longer than dens ; mucro short, with 
a prominent thick terminal tooth and two strong basal teeth 
(figs. 1, 6, 7). Colour slaty grey, the pigment somewhat scattered. 
Hairs on body short, a very few strong bristles on the tail segment. 
Locality. — Laurie Island, South Orkneys. Innumerable 
specimens on the sea-shore on the carcase of a penguin. 9th 
January 1904. 
The discovery of this insect in the Antarctic regions is of very 
great interest on account of its close relationship to the Arctic and 
sub- Arc tic /. Beselsii, Packard.* In the general build of the 
body and the structure of the spring — particularly the form of 
the mucro, with its three prominent claw-like teeth — these two 
species of Isotoma stand apart from all other members of the 
genus. I. Brucei is somewhat smaller than I. Beselsii , and has 
the spring, especially the dens, relatively longer and more slender ; 
the mucro relatively longer and narrower and its teeth weaker. 
These differences are, however, less apparent in young individuals. 
* Isotoma spitzbergensis, Lubbock. See Carpenter and Evans (1899), Schaffer 
(1900). The species is recorded by Wahlgren (1906) from Tierra del Fuego. 
