Dr. J. E. Gray’s Synopsis of American Squirrels. 415 
classification true for any large area be made on mineral cha- 
racter. 
But since the mutations in physical geography determine 
both the rock-material and the distribution of life, there is in 
them a philosophical basis for classification, which indicates the 
value alike of mineral character and of fossils. Yet classifica- 
tions, though made on the most fundamental considerations, 
can never be carried from a typical locality all over the world, 
because the world has neither life, nor mineral character, nor 
stability of physical geography in common with the typical 
locality. 
The proof, extension, and practical application of the cosmo- 
geny here sketched will be given in the first volume of the 
‘ Principles of Palzeontology,’ which is devoted to the dynamical 
geology of Britain. 
L.—Synopsis of the Species of American Squirrels in the Col- 
lection of the British Museum. By Dr. J. E. Gray, F.R.S., 
V.P.Z.S., &c. 
Tue species of American Squirrels are more difficult to define 
distinctly than those of Asia or Africa, arising from the various 
colours which the same species presents, even in individuals of the 
same family ; thus Bachmann states, “‘ Nothing is more common 
than to find the same litter composed of grey, black, and fox- 
coloured young.” 
Dr. Spencer Baird, in his well-studied essay on the Squirrels 
of North America, has shown that some species of the larger 
North-American Squirrels (as Sc. vudpinus) have a tendency to 
run into ferruginous varieties, and to have red bones, while 
other species (as Sc. carolinensis), of a yellow-grey colour, are 
very commonly affected with melanism, and have more or less 
black fur. 
Both Sc. vulpinus and Sc. carolinensis vary, on the under 
surface of the body, from pure white to rufous or black. 
Dr. Spencer Baird observes, as a general rule, that, where a 
squirrel exhibits any annulations of the hair on the fur of the 
throat or belly, it is a variety of some species which, in its 
normal form, has the under part either of a uniform white or 
reddish colour to the base, or only plumbeous at the roots. 
The hairiness of the soles of the feet varies, especially in the 
species which inhabit the northern region of America, or which 
have an extensive geographical range there. Dr. Spencer Baird 
describes specimens of Sciurus vulpinus “with (1) the soles 
naked, (2) the soles hairy between the pads nearly to the end 
