SCIURIOPTERUS. 121 
Fam. I. SCIURIDZ. 
1. SCIUROPTERUS. 
Sciuropterus, F. Cuvier, Ann. du Mus. x. p. 126 (1825). 
The true Squirrels which are provided with a flying expansion of the skin have been 
divided into two genera, Pteromys of George Cuvier, and Sciuropterus of his brother 
Frédéric—the members of the latter being distinguished by their smaller size, by their 
flat distichous tails, and by their molars remaining tuberculate through life, instead of 
being very soon worn down to a flat surface. In reviewing the genera of this order 
four years ago * I did not accord full generic rank to Sciuropterus ; but on further con- 
sideration I am now disposed to admit its validity, although some Indian species appear 
to be rather intermediate in their characters. Several minor cranial features, as 
the breadth of the nasals and the development of the postorbital processes, appear to 
be constant ; and the geographical distribution of the two forms is strikingly different ; 
for Pteromys is confined to the Oriental Region, whilst Sciwropterus, although extend- 
ing into India and Central America, is characteristic of the Palearctic and Nearctic 
faunee. 
Several species of North-American Flying-Squirrels have been described as distinct ; but 
Mr. J. A. Allen, in his invaluable monograph of the Sciuride of that continent}, has 
shown that only a northern and a southern form exist, and these he regards as varieties 
of the same species. They show a wonderful amount of geographical variation in size, 
specimens from the extreme north of the range being more than one half larger than 
examples from the extreme south. In the former the length of the head and body 
averages over 6 inches, while in the latter it is not more than 6, usually less than 
5:00 inches. There are also slight variations in colour, which, however, appear 
completely to intergrade. I have not sufficient material to form an independent opinion 
on the value of the differences between the two forms; but in any case the southern 
Flying-Squirrel will stand as Sciwropterus volucella (Pallas), while the northern, if 
regarded as distinct, will be S. hudsonius (Gmelin) t. 
1. Sciuropterus volucella. 
Mus volans, Linnzus, Syst. Nat. i. p. 85 (1776, ex Ray, nec Sciurus volans, Linnzus, tom. cit. 
_p. 88)$*. 
* P.Z.8. 1876, p. 76. 
t+ In Coues and Allen’s “ Monographs of North-American Rodentia,” Report U.S. Geol. Surv. of the Terri- 
— vol. xi. (1877). 
+ The works of Pallas (Nov. Sp. Glir.) and of Gmelin (Syst. Nat. i.) are both dated 1788; but the former is 
sosteas in the latter, and must consequently have been first published. 
§ Mus volans is founded on the Sciwrus americanus volans of Ray, while Sciwrus volans (ex Seba) is “the 
BIOL, CENTR.-AMER. Mamm. Vol. 1, June 1880. R 
