FAMILY. 
SCIURIDAE. 
Molars or rooted or rootless. 
Tibia and fibula distinct. 
No ante-orbital foramen in the anterior root of the zygoma, or else small and rounded. 
The family of Sciuridae as above constituted, and embracing the sub-families Sciurinae, My ox • 
inae, and Castorinae, has an extensive range: the first, throughout the world ; the second, in 
Europe, Asia, and Africa; the third, in Europe, Asia, and North America. The most promi¬ 
nent characters by which the sub-families may he distinguished are as follows : 
Sciurinae. —A distinct post-orbital process. Molars rooted, 
Myoxinae. —No post-orbital process. Molars rooted, No coecum. 
Castorinae .—No post-orbital process. Molars rootless, Aplodontia.) 
Sub-Family SCIURINAE. 
Molars, -jrf, (the anterior upper sometimes deciduous,) rooted; all nearly equal in size, except the anterior upper when there 
are five. A distinct post-orbital process of the frontal bone. The infra—or ante-orbital foramen very small, not piercing the 
broad plate of the zygoma, but anterior to this. 
The Sciurinae constitute a very natural group as characterized above, although there is a 
great diversity of appearance between the delicately formed and graceful arboreal true squirrels, 
at one end of the series, and the clumsy, squat, terrestrial marmots at the other. The compo¬ 
nent genera, Sciurus, Pteromys, Tamias, Spermophilus, and Arctomys , are all fully represented 
in North America by more species than are found in other parts of the world ; only one genus, 
Pteromys, having more species elsewhere than here. 
The muzzle is generally broad, owing to the development of the frontal and nasal bones. 
The acute process of the orbital edge of the frontal, which, pointing backwards, forms the 
hinder upper boundary of the orbit, is a peculiarity only found elsewhere, if I am not mistaken, 
among the hares. The palatine surface is large and broad, on the same plane from behind the 
incisors ; the incisive foramina far forwards and entirely in the intermaxillaries, seldom if ever 
passing backwards into the maxillary, which does not send forward a thin plate. The posterior 
margin of the palate passes considerably behind the last molar. There are two palatine fora¬ 
mina, either in the palato-maxillary suture or behind it. 
The zygomatic process of the upper maxillary is a broad thin plate which rises obliquely from 
the bone, is cylindrically concave in front, and has a broad concave notch at its lower edge, the 
postero-inferior edge of which is on a level with the bottom of the palate infernally, but exter¬ 
nally rises somewhat above this level; superiorly, this palate reaches up nearly to the top of 
the skull, articulating on the edge of the muzzle with the intermaxillary. There is, usually, 
