50 Proceedings of the Royal Physical Society. 
water; and; moreover, I do not remember having ever 
obtained Cypris incongruens in Loch Thom. 
It has, also, to be kept in mind that Ostracoda die very 
soon if exposed to the air, though they can survive a few 
days when enclosed in damp mud or vegetable matter. The 
egos of Entomostraca possess much greater vitality than the 
adult animals, and may, after a considerable lapse of time, be 
revived, as has been demonstrated by Professor G. O. Sars, 
who has raised Entomostraca from dried Australian mud; but 
the case I have described is different, in that the tank was 
cleaned out with a brush, so that, practically, no mud was left 
to form a protection, for either adults or eggs, from the 
scorching rays of the summer’s sun, yet nevertheless, here 
was a species of Ostracod in such immense numbers as to 
impart a yellow colour to the bottom of the tank. 
Cypris incongruens, though thus abundant and occurring 
in such out-of-the-way places, is yet by no means so 
ubiquitous as such examples would seem to imply. It is 
no uncommon experience to seek for it in vain where all 
the conditions favourable to its existence appear to be 
present. But this Ostracod is not only seemingly capricious 
in its selection of a habitat, it is also capable of living 
under very varied conditions, both as regards the temperature 
and the purity of the water. Mr David Robertson, the veteran 
Scottish naturalist, obtained it, many years ago,in a mill cooling- 
pond at Paisley. “Where the water issued into the pond, the 
temperature was 90° Fahr., and the surface water 80° Fahr. ; 
the Ostracods were found in the mud farthest from the heat.” 
On the other hand, my son obtained the same species at 
Greenock, in a ditch where there was very little water, and 
what there was of it, covered with a coating of ice. It may 
also be found in brackish, as well as in clean, water, and in. 
water so impure, from decomposing vegetable matter, as to 
be offensive both to sight and to smell. 
It will not be understood, though I have thus tried to 
illustrate the vagrant distribution of a particular species, 
that such a distribution is exceptional, or confined to that 
species. It may be observed, more or less, in many other 
species. belonging to the group. It is this kind of dis- 
tribution that makes the study of these minute organisms 
