Dr. J. E. Gvay'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 classitication, 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 Palaeontology,' 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.U.S., 

 V.P.Z.S., &c. 



The 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 Squiri'els (as Sc. vulpinus) 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 Ban-d 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 

 nakfd, (2) the soles hairy between the pads nearly to the end 



