NATURAL HISTORY. 
ROOM III.] 
l'2o 
The fourth order or Glirine Mammalia (Glires, Cases 29 to 43) 
are easily known, because they have only two strong cutting teeth in 
each jaw, which are separated from the grinders by a space. They 
have no canines, and the condyles of the jaws are longitudinal, which 
enable them to move their jaw from side to side and thus triturate their 
food. Their toes are distinct, with small conical claws, and the thumb 
is sometimes rudimentary. 
Some have a scaly or spinose tail, and there are scattered spines 
or larger hair intermixed with the softer fur. 
The family of Mice (Murids) have a more or less elongate tail, 
covered with whorls of scales; the fur is soft, w T ith scattered longer hairs 
or flat channelled striated spines ; the limbs are proportionate, the front 
thumb wart-like; the lower cutting teeth are awl-shaped, the grinders of 
the upper jaw shelve backward, and the lower ones forward; the ears 
and eyes are distinct. The Murina have separate toes, a wart-like 
thumb to the fore feet, and tubercular grinders. Many of these have 
three grinders in each jaw T , as Mus and Pseudonys , which have an 
elongated tapering tail and no cheek pouch; Cricetomys has a long, 
and Cricetus a shorter tail and cheek pouches, and Hapalotis has a small 
pencil of longer hair, somewhat like a jerboa, at the tip of the tail. 
Others, as Perognathus, have four grinders in each jaw. Other genera 
have the crown of the grinders flat, as if they were truncated; some 
of them have the crown cut in on opposite sides, as the genera Mys- 
tromys, Rhombomys , Psavimonys , Meriones , and Euryotes ( Otomys, 
F. Cuv.) The remainder have the crown cut in with lobes in 
the enamel alternately first on one side and then on the other, as 
Sigmodon , Neotoma , Elismodon, Peithrodon , My odes, and Arvicola. 
The genus Sminthus is peculiar for having four grinding teeth in 
the upper and three in the lower jaw. Others, as Saccomys , Hetero- 
mys, and Dipodomys, have four teeth in each jaw, they also have a 
membranaceous pouch on each side of the face, opening externally on 
the cheek; the tw 7 o former have elongated tapering tails, but the first is 
covered with soft fur ; and Heteromys with flat channelled spines, and 
Dipodomys has a depressed bunch of soft hair on the tip of the tail. 
The Castorina have five toes on the front foot, and the hinder foot en¬ 
larged and more or less distinctly w^ebbed; they live chiefly at the 
side of the water, as the Beaver ( Castor) of the northern regions, which 
has four grinders in each jaw, and a broad depressed scaly tail; the 
Corypus ( Myopotamus ) of Central America, which has the same 
number of grinders, and a cylindrical tapering tail; both of these have 
rooted, rounded, alternately plaited teeth; the Ondatra {Fiber) of 
North America, which has three flat crowned grinders in each jaw 
and a tapering tail, and elongated alternately plaited rootless teeth ; and 
the Hydronys of Australia, which has the same kind of tail, but only 
two rooted grinders in each jaw with opposite folds. The Psammoryc- 
tina have four grinders in each jaw; the angle of the lower jaw produced 
into a hook, the ears are small or moderate. They are only found in the 
warmer parts of America. The Echimyna have four grinders in each 
jaw, and the angle of the lower jaw T is produced into an elongated pro¬ 
cess ; their limbs are proportionate, and the ears moderate ; they are pe¬ 
culiar to Central America. Some, as Habrocoma , Octodon, and Psarn- 
moryctes, have very soft fur; Capromys and Aulacodus have bristly 
