654 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
Squirrels of Europe and North America from the others under the name 
Sciuropterus, and pointed out the important differences in the form of the 
skull and in the structure of the teeth that mark the two groups. By subse- 
quent writers, the two groups, Pferomys and Sciuropterus, so well charac- 
terized by M. F. Cuvier, have not been generally adopted, and Sciuropterus, 
when recognized at all, has usually been accorded only the rank of a sub- 
genus of Pteromys. The two groups, however, differ widely, not only in size 
and other external features, but in cranial and dental characters. The species of 
Pteromys are generally exceeded in size among the Sciuride only by those of 
Arctomys; they also differ from the Sciwropteri in having the tail long, round, 
and bushy, instead of distichous and laterally expanded. In Péeromys, the 
frontal region of the skull is depressed; the nasal bones are broad and 
swollen, and the postorbital processes are greatly developed, being relatively 
almost as large as in Arctomys. The large size of the postorbital processes 
and the depression of the interorbital region give to the dorsal aspect of the 
skull some resemblance to the skull of Arctomys. In respect to the dentition, 
the four posterior upper grinding teeth, instead of being subequal in size, as 
in Sciuropterus, are very unequally developed, the last being less than half 
the size of the three preceding.* ‘The structure of the grinding teeth is also 
wholly different from that met with in any other genus of this family, the 
triturating surface not distantly resembling that presented by Castor, in con- 
sequence of the deep infolding of the enamel border of the tooth. There 
are, howeyer, small isolated rings of enamel in the spaces between the deep 
infoldings of the enamel of the border of the crown, somewhat as in worn 
teeth of Erethizon. 
The species of Pteromys are restricted in their distribution to Southern 
Asia and the Indian Archipelago; those of Sciuropterus range over the colder 
portions of the northern hemisphere, extending southward to intertropical 
latitudes. ‘ 
*In F. Cuvier’s figure of the dentition of Pteromys (Dents des Mammiferes, pl. lvii), drawn from 
“Sciurus petaurista Pall.”, the second premolar (first large grinding tooth) is also much smaller than 
either of the two immediately succeeding. In Brandt’s figures of the skull of “ Pleromys nitidus” (Mém. 
de Acad. Imp. des Sci. de Saint Pétersb. 6e ser., Sci. Nat. t. vii, pl. i, figs. 1-7), however, the second 
premolar is the largest of the grinding series, and I find this to be so in skulls of this species in the 
Museum of Comparative Zodlogy. 
