RODENTIA. 
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on its outer side. Descending ramus very large and flat. Tibia and fibula anchylosed below. 
This family contains but two genera, Lepus and Lagomys, both found in North America ; the 
former represented by many species, the latter by a single one. 
II. Hystricidae. —Incisors molars with or without roots. The snout clothed with short 
hairs. The skull with a large ante-orbital opening in the zygomatic process of the upper maxil¬ 
lary ; the lower jaw with the angular portion joined to the outer, not the under surface of the 
bony covering of the inferior incisor ; tibia and fibula distinct. 
Of the six sub-families of this group, Hystricina, Dasyproctina, Echymyina, Octodontina , 
Chinchillina, and Caviina, North America possesses but a single genus, Erethizon, belonging to 
the Hystricina above mentioned. Mexico has, however a second in Cercolabes. The entire 
group belongs to America, except the few species of true Hystricina , which are found in Asia 
and Africa, and some genera of Echymyina. 
III. Muridae. —Incisors molars, rooted or rootless, ^ to usually ; they diminish 
posteriorly. A conspicuous opening on the zygomatic process of the upper jaw, which is a 
vertical slit, widened above, except in the Dipodina, where it is a more expanded aperture. 
The coronoid and condyloid processes and descending ramus distinct and well developed, 
situated in nearly the same plane, the latter more or less twisted. 
This family abounds in North American representatives, belonging to the sub-families of 
Dipodina, Murina , and Arvicolina. The genera are, Jaculus for the first; Hesperomys, Reith- 
rodon, Neotoma, and Sigmodon for the second ; and Arvicola, Myodes, and Fiber for the third. 
The Dipodina in some points exhibit considerable differences from the other sub-families, and 
may prove to be of independent rank. 
IV. Saccomyidae. —Incisors molars quite simple, and mostly rootless. No perforation 
or foramen in the zygomatic process of the upper maxillary. The auditory bullae very far 
back, sometimes forming part of the back of the head. The condyloid process is small; the 
descending ramus is twisted nearly horizontally at its upper angle, so that when viewed from 
above a great portion is visible with a tubercle covering tbe end of tbe incisor between it and 
the condyle. 
On each side of the head are cheek pouches, opening externally to the mouth. Five toes to 
each foot; the fore feet usually with broader palms and longer claws than the hinder ones. 
This very curious family is eminently characterized by its cheek pouches, which are exterior 
to the mouth entirely, instead of opening from within it, as in Tamias, Spermophilus, &c. The 
peculiarities of these pouches will be discussed more particularly hereafter. 
The genera of this family are confined to North and Central America. Geomys, Thomomys, 
Dipodomys, and Perognatus occurring within the United States; Heteromys and Saccomys, on 
the other hand, being extra limital. In this country, species of this family are, however, 
confined mainly to the region west of the Mississippi. 
V. Sciuridae. —Incisors molars in many American forms ; nearly equal in size, 
except the anterior, when five are present. Molars mostly rooted. No foramen in the zygo¬ 
matic process of the superior maxillary ; the ante-orbital foramen very small, and appearing far 
forward. A distinct post-orbital process in the typical forms. Lower jaw with the descending 
ramus broad and inflected at its lower angle. 
Among the North American species of this type, there are several distinct sub-families, which 
will be referred to hereafter. The genera are Sciurus, Pteromys, Tamias, Spermophilus, Cynomys, 
Arctomys, Aplodontia, and Castor. 
