148 



nothing very remarkable or dissimilar to the arrangement found in other 

 orders. As vegetable feeders the intestines are long, but the stomach is 

 simiDle and unlike the Ruminantia. 



The bony skeleton is light, and suited to the great activity dis- 

 played by this order of Mammals, and in nearly all the members of 

 the order the hind limbs are very much longer than the fore, and 

 this is especially observable in those animals like the jerboas, vifhich 

 proceed by a series of leaps. In them the metatarsal bones are very much 

 elongated and coalesce into one, giving almost the appearance of the leg of » 

 bird. The most important pecidiarity of structure, however, in this order, con- 

 sists in the incisor teeth. These, as I have ah-eady mentioned, are always two in 

 number in both jaws, and are seated in very deei> sockets. The outer surface 

 of the teeth is covered by a very hard enamel, and the body of the teeth is 

 formetl of a hard dentine, this latter substance being, however, much less hard 

 than the enamel ; also, the enamel on the outside of the incisor teeth is con - 

 siderably harder than that on the inside. The result of this formation is that 

 by the continual gnawing at hard substances like wood, nuts, &c., the softer 

 parts of the tooth are worn away more quickly than the harder, and the out- 

 side enamel becomes sharpened into the form of a chisel, admirably adapted 

 for cutting into hard surfaces. As by the constant trituration in bitmg the in- 

 cisor teeth would soon become worn away, if not renewed, there is at the 

 base of these teeth which I have already said is deep in the jaw, i^rovision for 

 a constant renewal or growth of the tooth, so that as the tooth is constantly 

 worn away at the point it is as constantly added to at the base. It sometimes 

 happens that from the loss of one of the upper or lower teeth the opposing 

 tooth grows to a great length, and continues its direction of gi-owth in the same 

 arc of a circle as it exists in the jaw, and the result is that the animal 

 is quite unable to open its mouth wide enough to use the tooth and so some- 

 times dies of starvation. The molar teeth are usually few in number and 

 separated from the two incisors by a considerable gap. The jaws are so 

 attached to the skull as to allow of very ample lateral movement. In the 

 genera Lepus (hares and rabbits), and Cavia (guinea pigs), the clavicles are 

 (mly imperfectly developed. In all the other British genera belonging to this 

 order the clavicles are well developed, and in nearly every species considerable 

 rotatory motion is permitted to the fore limbs which are extensively used as 

 instruments for grasping. I pass on now to notice the fonnation of the brain 

 in this order. This organ in the order Rodentia presents an intermediate 

 appearance between the brains of the nonplacental animals (Marsupials and 

 JMonotremes), which are usually placed by zoologists below them, and the 

 higher orders such as Carnivora, Ruminantia, and Quadnimana. In the order 

 Rodentia, and the same remarks apply pretty generally to the three orders, 

 Bruta, Cheiroptera, and Insectivora, the cerebral lobes are nearly quite 

 smooth, and not traversed as in other animals by furrows; their 

 exteit is greater than in the Marsupials and Monotremes, but they 



