054 



MOXOGRAPnS OP NORTH AMERICAN RODENTIA. 





i 



4m :i 



Squirrels of Europe and North America from the others under the name 

 Sciuropferus, and pointed out the important differences in the form of the 

 sivuU and in the structure of tiic teeth that mark the two groups By subse- 

 quent writers, the two groups, Pleromys and Sciurop/ervx, so well charac- 

 terized by M. F. Cuvier, have not been generally adopted, and Sciuroptfrus, 

 when recognized at all, has usually been accorded only the rank of a sub- 

 genus of Pteromya. The two groups, however, differ widely, not only in size 

 and other external features, but in cranial and dental characters. The species of 

 Pleromys are generally exceeded in size among i\^e Sciuridte only by those of 

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

 and bushy, instead of distichous and laterally expanded. In Pleromys, 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, 

 tiie 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. Cnvier's flgnrn of the dentition of Pieromyt (Denta dc* Hammlf^raa, pi. Wti), drawn fitim 

 " Sciunit ptiaHrUla Pall.", the iiecond premolar (Brat large grinding tooth) ia aliw mnoh amaller than 

 cither of the two immediately snoceeding. In Brandt's figures of the aknll of " Pleromji$ niUdui" (M(Sni. 

 de I'Acod. Imp. dea Scl. de Saint P^terab. 6e aer., 8oi, Nat. t. vii, p|. i, flga. 1-7), however, the aeoond 

 premolar ia the largest of the grinding aeries, and I find this to be so in skalls of this apecies in the 

 Museom oi Comparative Zodlogy. 



