286 U. Ss. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
PTEROMYS, Cuvier. 
Pteromys, Cuvier, ‘‘ Legons d’Anatomie, I, 1800.’’—Aud. & Bach., N. Am, Quad. I, 1849, 132. 
A densely furred membrane, extending laterally from the sides between the fore and hind feet. Permanent molars five. 
Muzzle short. A deep supra-orbital notch, 
The genus Pteromys is most remarkably characterized by the membranous expansion of the 
sides between the fore and hind feet, by means of which the animal is enabled to glide through 
the air from one tree to another, supported as by a parachute. Anteriorly this membrane is 
attached, in part, to a slender bone springing from the hand. 
There are two distinct sections of the genus Pteromys, which may be characterized as follows: 
1. Pteromys. Tail rounded. Molars complicated, Post-orbital process very large. 
2. Sciuropterus. Tail depressed and flattened. Molars simple, as in other squirrels, 
All the American and European species belong to the second division. 
In general appearance the skull of the American Pteromys is very similar to that of the short- 
nosed squirrels, like Sciwrus hudsonius. The post-orbital processes are a little longer and more 
recurved. The supra-orbital notch is very deep, so that the width of the frontal bone at that 
point is considerably less than behind the post-orbital processes. The upward bend of the 
zygoma at the zygomatic plate is more abrupt than in any other Sciurinae. This plate itself is 
nearly plain, and is not perforated as in Zamias, the ante-orbital foramen being more anterior 
and vertically elongated. There are always five upper molars, the anterior quite rudimentary, 
and barely in advance of the notch of the posterior edge of the zygomatic plate. The inner 
outlines of the alveoli are straight and parallel. 
The skull is distinguishable from that of Sciwrus by the depth and size of the supra-orbital 
notch ; agreeing, however, with that of S. hudsonius in the shortness of the muzzle. It is 
much fuller and more convex than that of Zamias, with a shorter muzzle, zygomatic plate not 
perforated, and five upper molars. The auditory bullae are very large. 
I regret that I am unable to describe any species but P. volucella in detail ; the flying squir- 
rels being more poorly represented in the collection of the western expeditions than any other 
mammals. Itis not improbable, however, that one, at least, of the species of Audubon and Bach- 
man may be a synonym, as the proportional length of the tail and the amount. of expansion of 
the flying wing membrane, are not to be determined with any degree of precision in dried 
specimens of animals so readily over-stretched as the flying squirrel. 
I have not been able to make any comparison of European flying squirrels with the American, 
for want of specimens. 
PTEROMYS VOLUCELLA. 
Flying Squirrel. 
Sciurus volucclla, Pauuas, Glires, 1778, 351, 353, 359. 
Guetin, Syst. Nat. I, 1788, 155. 
Scurener, Saugthiere, IV, 1792, 808; tab. ccexxii. 
Suaw, Gen. Zoology, II, 1801, 155; tab. clv. 
Pleromys volucella, DesMarest, Mamm. Il, 1822, 343. 
Haran, Fauna Americana, 1825, 187. 
Grirrirn’s, Cuvier, V, 1827, 259. 
Fiscuer, Synopsis, 1829, 365. 
- Gopman, American Natural History, IJ, 146. 
‘* Bennett, Gardens and Menagerie Zool. Soc., I, 185.” 
