1240 THE RODENTS 



continue to grow on uninterruptedly 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 dis- 

 tinct 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 pro- 

 ducing a more or. less complicated pattern when worn. On the other hand, the 

 permanently-growing rootless molars always have complex crowns, which are some- 

 times interpenetrated by lateral infoldings of the enamel, and at others are divided 

 into 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 inward 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 contain- 

 ing 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. 



The Rodents have a wider distribution than any other group of ter- 

 d H b't rest " a l Mammals, being found in all parts of the globe except the ex- 

 treme polar regions. With the exception of the bats, they include 

 the only representatives of the placental Mammals which are undoubtedly indige- 

 nous 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, 

 although it has been suggested that they are derived directly from the Marsupials. 



