i 
HYSTRICIDA.—ERETHIZON. 387 
stiff hairs and spines, and on the 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 
in 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 Evrethizon with that of Hystrix shows that the 
differences are far greater than the resemblances. When seen from above, the 
skull of Evethizon 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 zy go- 
matic arch is similarly widely expanded laterally. Lrethizon, however, lacks 
the greatly developed postorbital process of the froutal bones seen in Arctomys, 
and of course differs greatly in general details of structure. Hystriz, 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 Evethizon.* '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 Lrethizon, 
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 Evrethizon, in which both are very large. The 
*TIn respect to the inflation of the skull, Synetheres is about intermediate between Hysirix and 
Brethizon, the muzzle being wide, and the frontal region abruptly and greatly swollen. The molar series 
are also less convergent than in Lrethizon. 
