0.*J4 MONOC.RArns of north American uodentia. 



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, Pteromys and Sciuropterus^ so well charac- 

 terized hy M. F. Cuvier, have not been generally adopted, and Sciuropterus, 

 when recognized at all, has usually heen 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 Sciuridce only by those of 

 Arctomys; they also differ from the Sciuropteri in having the tail long, round, 

 and bushy, instead of distichous and laterally expanded. In Pteromys, 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, however, 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, pi. lvii), drawn from 

 "Sciums pelaurista Pall.", the second premolar (first large grinding tooth) is also much smaller than 

 cither of the two immediately succeeding. In Brandt's figures of the skull of " Pteromys nitidus" (Mem. 

 do l'Acad. Imp. des Sci. de Saint PtStcrsb. 6e ser., Sci. Nat. t. vii, pi. 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 oa Comparative Zoology. 



