386 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
(Sv) 
hind feet, and in havinga short, spiny, non-prehensile tail. The South Ameri- 
can genus Chetomys differs from all of the others in the conformation of the 
skull; the malar bone is developed toa remarkable degree, more than in 
any other Rodent, excepting Celogenys, the postorbital process of which 
nearly joins that of the frontal, so as to almost wholly separate the orbital 
and temporal fosse. The portion of the palate between the molar teeth is 
also longer and narrower, and the temporal ridges are more highly developed, 
while the molars themselves present important differences. The tail is 
lengthened and rat-like, scantily clothed with hair, and scaly, though to some 
degree prehensile. The genera Synetheres and Sphingurus differ somewhat 
from Erethizon in their cranial and dental characters, and have the tail strongly 
prehensile, while the feet (as also in Chetomys) are all four-toed. Synetheres 
differs from Sphingurus in the broad, highly arched form of the frontal region 
of the skull, and in the greater development of the spines. Chetomys has 
but a single species (C. subspinosus), which is confined to the northern and 
central portions of Brazil. Sphingurus and Synetheres have each several com- 
monly recognized species, which are distributed throughout the greater part 
of South and Central America, from Paraguay to Southeastern Mexico and 
the West Indies; none, however, occur west of the Andes. All the species 
- are of rather small size in comparison with the representatives of Evethizon ; 
the body rarely exceeding eighteen inches in length. 
Considerable variation in the form of the frontal region of the skull 
occurs among both the Hystricine and the Synetherine. In this respect, 
Synetheres approaches Hystriz, while the Old World genus Atherura, with 
its flattened skull, more resembles Erethizon. Even different species of 
Hystrix vary considerably in respect to the inflation and convexity of this 
portion of the cranium, showing that this is a feature of no very great taxo- 
nomic importance, though giving rise to striking differences in respect to the 
dorsal contour of the skull. 
GENUS ERETHIZON F. Cuv. 
North American Porcupines. 
Hystrix, in part, of earlier writers. 
Erethizon ¥. CuvIER, Mém. du Mus., ix, 1822, 426, 432. 
Echinoprocta Gray, Proc. Zod]. Soc. Lond., 1865, 321. 
Toes four in front and five behind, all armed with strong, curved claws. 
Tail short, thick, depressed, non-prehensile, covered above at the base with 
