3bG MONOGRAPHS OF NORTH AMERICAN KODENTIA. 



hind feet, and in having a short, spiny, non-prehensile tail. The South Ameri- 

 can genus Chcetomys differs from all of the others in the conformation of the 



skull; the malar bone is developed to a remarkable degree, more than in 

 any other Rodent, excepting Ccelogrnys, (he postorbital process of which 

 nearly joins that of the frontal, so as to almost wholly separate the orbital 

 and temporal fossae. 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 Chcetomys) 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. Chatomys 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 Erethizon; 

 the body rarely exceeding eighteen inches in length. 



Considerable variation in the form of the frontal region of the skull 

 occurs among both the Hyslricince and the Synetherince. In this respect, 

 Synetheres approaches Hystrix, while the Old World genus Atherura, with 

 its flattened skull, more resembles Erethizon. Even different species of 

 Hyst/ix 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 F. Cuvikr, Mem. du Mus., ix, 1822, 426, 432. 



Echinoprocla Gray, Proc. Zool. Soc. Lond., 18C5, 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 



