GENERAL CHARACTERS. 67 
of these teeth be broken away during life, the corresponding tooth in the opposite 
jaw, having nothing to check its growth by wear, will continue to grow on un- 
interruptedly till it forms nearly a circle, when its point will enter the jaw of its 
unfortunate owner, and probably lead to a lingering death. Most museums possess 
specimens of the skulls of Rodents with such abnormally-developed teeth. 
The cheek-teeth of Rodents may be either provided with distinct roots, or 
may grow throughout life and never develop such roots. In the cases of those 
with distinct roots, the crowns may carry mere simple tubercles; but in other 
instances they may have their summits or sides penetrated by infoldings of the 
enamel, thus producing a more or less complicated pattern when worn. On the 
other hand, the permanently-growing rootless molars always have complex crowns, 
which are sometimes interpenetrated by lateral infoldings of the enamel, and at 
others are divided into a series of parallel transverse plates. There is, indeed, a 
precise parallelism in this respect between the molar teeth of the Rodents and 
those of the mastodons and elephants; the molar tooth of a mouse, which has 
distinct roots, and a low crown with simple cusps, being exactly comparable to 
that of a mastodon, whereas the high-crowned, laminated, and rootless molar of 
a guinea-pig corresponds as closely with that of a modern elephant. Rodents are 
always furnished with milk-teeth, which, however, in some instances are shed 
before birth. 
The Rodents present a peculiarity in the structure of the mouth, 
which is quite unknown in any other mammals. In examining the 
mouth of any one of these animals—say a rabbit—it will be found that behind the 
upper front teeth the outer hairy skin of the face is continued inwards into the 
sides of the mouth, which by this means is divided into two distinct chambers, 
communicating with one another through a comparatively narrow orifice ; the first 
chamber containing only the front teeth, while the cheek-teeth are included in the 
second chamber. It appears that this arrangement is designed to prevent the 
entrance of extraneous substances into the true cavity of the mouth when the 
creatures are engaged in their characteristic operation of gnawing. In addition to 
this peculiarity, the whole of the inside of the cheeks in the hares and rabbits is 
covered with hair; while the pouched rats and hamsters have large pouches inside 
the cheeks, which are also lined with hairy skin. On the other hand, the gophers 
have pouches in the cheeks which open externally instead of internally. 
Mouth. 
Distribution and The Rodents have a wider distribution than any other group of 
Habits. terrestrial mammals, being found in all parts of the globe except the 
extreme polar regions. With the exception of the bats, they include the only 
representatives of the placental mammals which are undoubtedly indigenous to 
Australia. They are, however, but poorly represented in that country, and likewise 
in the great island of Madagascar. On the other hand, the order attains an 
extraordinary development in South America, where its largest representatives 
occur, and which may be looked upon as its headquarters. In time, the Rodents 
certainly date from the upper portion of the Eocene division of the Tertiary period ; 
but the whole of the extinct forms hitherto discovered present all the essential 
peculiarities of the existing representatives of the group, and we are, therefore, at 
present almost completely in the dark as to their relationship to other orders, 
