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 abnormalh'-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, 

 Mouth. J- , T ... 



which is quite unknown 111 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. 



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, 



