Famiy HYSTRICID AE. 
By several recent systematic writers, the New World Poreupines have 
been widely separated from their Old World relatives; these authors placing 
them in different families. The two forms that differ most widely in cranial 
characters are Erethizon and Hystrix, especially in respect to the degree of 
inflation of the skull; but the other forms, in a measure, bridge over the wide 
gap existing between these two types in respect to this feature. The Ameri- 
ean Porcupines form a group collectively separable as a subfamily, by quite 
tangible characters, from the Porcupines of the Old World. . While the 
former are arboreal, the latter are terrestrial, and the two types present modi- 
fications of structure adapting them to these widely different modes of life. 
There is, however, running through the whole, a strong degree of resemblance. 
The Synetherine,* or the American Porcupines, differ from the Old World 
Porcupines in the form of the skull; in having the clavicles perfect; in the 
tail being generally (not, however, in Erethizon) more or less prehensile; in 
the molar teeth being fully rooted, and placed in more or less converging 
series; in not having five toes to all the feet (generally only four both before 
and behind) ; in the soles being tuberculated instead of smooth; in the upper 
lip being undivided by a vertical groove; in the form of the lachrymal bone ; 
and in various other more or less important osteological characters. 
The subfamily Synethering is most numerously represented in South 
America, where occur three of its four genera, namely, Chetomys, Synetheres, 
and Sphingurus;t+ the fourth, Evrethizon, being its only representative in 
North America north of Mexico. While these genera have many features 
in common, and constitute a very natural and well circumscribed group, 
Erethizon differs from the others in having five instead of four toes on the 
* Synetherina Gervais, Zool. et Paléont. frang., 1848-52, p. 18; Sphingurine Alston, Proc. Zo6l. Soe. 
Lond., 1876, 93; = Cercolabine of the family Spalacopodida of Lilljeborg (Systematisk @fversigt af de 
Gnagande Diiggdjuren, Glires, 1866, p. 51) and of Gill (Arrang. Families of Mammals, p. 22). 
t Synetheres and Sphingurus F. Cuvier, 1822, = Cercolabes Brandt, 1855. 
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