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. 

 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, 



