HYSTKICID^— ERETHIZON. 387 



stiff hairs and spines, and on (lie sides, at the apex, and beneath with thick 

 rigid bristles. Size large. Limbs short and strong. 



As already stated, Erethizon differs from the other American Porcupines 

 iu possessing five toes to the hind feet, all armed with strong claws, and in 

 its short, thick, non-prehensile tail. It also differs in its more approximated 

 nostrils and in its large size. From the Old World Porcupines, with which 

 it was formerly for a long time generically associated, in common with all 

 the New World species, it presents many important points of difference. 

 A comparison of the skull of Erethizon with that of Hystrix shows that the 

 differences are far greater than the resemblances. When seen from above, the 

 skull of Erethizon bears, in its general form, a striking similarity to that of Arc- 

 tomys, the dorsal outline being nearly straight and the frontal region depressed 

 and flat ; the nasals are of about the same relative size and shape, and the zygo- 

 matic arch is similarly widely expanded laterally. Erethizon, however, lacks 

 the greatly developed postorbital process of the frontal bones seen in Arctomys, 

 and of course differs greatly in general details of structure. Hystrix, on the 

 other hand, is exceptional among Rodents for the great development of the 

 nasal and frontal elements of the skull, consequent upon the enormous size 

 of the nasal and frontal sinuses. The frontals are hence twice the size of the 

 parietals instead of being much smaller, as in Erethizon, while the nasals are 

 still more remarkably developed, they rapidly widening posteriorly and 

 extending as far back as the middle of the zygomatic arch. This results in 

 an interorbital breadth almost unparalleled among ordinary Rodents, equaling 

 one-half of the total length of the skull instead of less than one-third, as in 

 Erethizon, and gives to the skull a high, greatly swollen, convex, dorsal outline 

 instead of the straight, flat one seen in Erethizon * The zygomatic process 

 of the maxillary is greatly expanded and thickened, forming an immense 

 oblique pier, about one-half as broad as long, for the attachment of the zygo- 

 matic arch, instead of being a rather slender, thin process, as in Erethizon, 

 while the slender horizontal process of the same bone, which bounds the 

 lower part of the anteorbital fossa, appears like a second small zygomatic 

 arch. There hence results a structure as different from that of Erethizon, 

 as can well be imagined. The orbital fossa is small, and the temporal 

 many times smaller than in Erethizon, in which both are very large. The 



* In respect to the inflation of the skull, Synetheres is about intermediate between Hystrix and 

 Erethizon, the muzzle being wide, and the frontal region abruptly and greatly swollen. The molar series 

 are also less convergent than in Erethizon. 



