CHARACTERS OF THE RODEXT ORDER. 



of this structure of the incisors is easily understood. In the action of gnawing, the dentine, which 

 forms the greater part of the tooth, is more easily abraded than the harder enamel, which is tln:s left 

 as a sharp front edge, to which the mass of dentine behind it, being worn away into a bevelled surface, 

 gives the necessary firmness and support, the whole forming a chisel-like instrument, constructed 

 precisely on the principle of those tools in. which a thin plate of hard steel forms the cutting edge, and 

 is stiffened by a thicker bevelled plate of softer iron. 



The canine teeth are entirely deficient, and behind the incisors we find on each side a toothless gap 

 of considerable extent (see figures p. 82), beyond which come the gi-inding teeth. In these it is difficult 

 to recognise any distinction of molars and pre-molars; the whole series presents nearly the same structural 

 characters, and for all practical pui-poses we may speak of them as molars, although some zoologists 

 prefer to regard the three hindmost teeth on each side as true molars, and any others that may be 

 present as premolars. In one genus (Hydromys) the number of grinding teeth is reduced to two on 

 each side in each jaw ; in a great proportion of the species the number is three ; others have four or 

 five grinders on each side, either in one or both jaws (usually one more in the upper series) ; and the 

 largest number is possessed by the Hares and Rabbits, in which the upper jaw has six and the lower five 

 grinders.* The grinders are sometimes furnished with true roots, but are more commonly open belov/, 

 and provided, like the incisors, with a permanent pulp. They are sometimes tubercular, at least in 

 youth, but generally show a flat, worn surface with transverse bands, or re-entering folds, and sometimes 

 cylinders of enamel, which display a great variety of patterns. Sometimes the enamel is confined to 

 the surface of the tooth ; in other cases each tooth is, as it were, made up of two or more variously- 

 shaped tubular portions of enamel, filled up with dentine. Curiously enough, this structure of the 

 grinders, especially the arrangement of the transverse ridges and plates of enamel in these little animals, 

 reminds us strongly of the characters of the molars of the gigantic Proboscidea, in which, moreover, 

 the incisors also are represented by the permanently-growing tvisks. 



The articulation of the lower jaw with the skull is peculiar, and in special relation to the armature 

 of teeth which we have described. Instead of articulating freely, as in man and many herbivorous 

 mammals, by which provision is made for a sort of rotatory action of the molars, or by a regular trans- 

 verse hinge-joint, as in the Carnivora, the articulating surfaces are elongated in a direction parallel to 

 the middle line of the skull, an ar- 

 rangement which, like that occurring in 

 Carnivora, has the effect of preventing 

 much lateral movement of the jaw ; 

 but, at the same time, the pits with 

 which the jaw articulates are open in 

 front, so that the jaw is allowed a 

 certain amount of play, backwards and 

 forwards. This motion greatly increases 

 the gnawing power of the large incisor 

 teeth. 



The head in the Rodents is gene- 

 rally of small or moderate size in pro- 

 portion to the body, and the skull is 

 usually rather elongated, and flattened 

 on the upper surface. The tympanic 

 Imllae are generally of considerable size ; 

 the zygomatic arch is in nearly all cases 

 well developed; but the orbits of the 

 eyes are never closed behind, and only 

 in certain families is there even a small process of the zygomatic arch behind the orbits, as an 

 indication of possible closure. Of the vertebral column we need only say that the lumbar vertebrae 

 are remarkable for possessing large transverse processes directed forwards, and that the tail varies 



* The genus Hdiophclius among the Mole Rats is described as having six molars on each side in both jaws ; but 

 the number in this genus appears to be variable, the sixth molar being often undeveloped. 



SKELETON OP THE RABBIT. 



