THE SIMPLE-TOOTHED RODENTS. 



85 



proposed fifty years ago by Professor Gervais, and characterised by the number of incisor teeth. 

 The first of these sub-orders, which includes by far the majority of the Rodents, is formed by those 

 species which never at any period of their lives possess more than two incisors in the upper jaw, 

 and have the enamel on these strictly confined to the front surface of the teeth. These are 

 denominated SIHPLICIDENTATA, or SIMPLE-TOOTHED RODENTS. In the second group, which includes 

 only the Hares, Rabbits, and Calling Hares, we have those species which in the adult state possess 

 four incisors in the upper jaw, namely, two large and efficient teeth, and behind these two small, 

 almost rudimentary incisors (see figure of the dentition of the Hare on p. 82). These are called 

 DOUBLE-TOOTHED RODENTS, or DUPLICIDENTATA. 



SUB-ORDER I. SIMPLE-TOOTHED RODENTS. 



Besides the characters derived from the number of incisor teeth above mentioned, several other 

 peculiarities of structure seem to show the existence of a decided difference between the Simple-toothed 

 and Double-toothed Rodents ; but most of these are of a rather abstruse nature, and need not be 

 noticed here, the most important additional distinctive characters of the former being that the 

 bony palate is well developed, and that the fibula does not articulate with the calcaneum, or heel-bone ; 

 whereas in the Double-toothed Rodents the palate is reduced to a mere bridge between the portions 

 of the upper jaw in which the teeth are inserted, and the fibula does articulate with the heel-bone. 



The Simplicidentata include a great number of families, and various attempts have been made 

 to group these under larger heads ; but it must be confessed that, owing to the way in which the 

 families approach one another, it is difficult to bring them together in sections capable of being very 

 strictly defined. Certain broad principles of relationship are, however, generally recognised, and 

 Mr. Alston represented these by placing the Simple-toothed families under three great sections, the 

 first indicated by Mr. Waterhouse the Squirrel-like, Mouse-like, and Porcupine-like Rodents. 



The Squirrel-like Rodents have four molars on each side in the lower jaw, and either four or five 

 in the upper. When the latter number is present, the foremost tooth is smaller than the rest. The 

 fibula remains as a distinct bone through life, and is usually quite free, although sometimes attached 

 to the tibia at the extremity. The upper lip is usually cleft, the muffle is small and naked, and the 

 nostrils are comma-shaped, with the rounded part 

 above. The zygomatic arch is formed chiefly by the 

 process of the malar bone, which is not supported 

 below by a continuation of the zygomatic process of 

 the maxillary. The collar-bones are perfect. The 

 tail is usually cylindrical and hairy. 



The Mouse-like Rodents agree with the pre- 

 ceding in the characters of the upper lip, muffle, and 

 nostrils, but they have the tibia and fibula completely 

 united for at least the last third of their length. The 

 zygomatic arch is slender, and the malar process rarely 

 extends so far forward as in the preceding group 

 and is generally supported below by a continuatio 

 of the maxillary process. The collar-bones are pel 

 feet, except in one very small family; and the tai 

 is cylindrical, and although sometimes hairy, more 

 commonly covered with scales arranged in rings. 

 The number of molar teeth in this section varies from 

 three to six * on each side in each jaw, but three is 

 the most usual number. 



The Porcupine-like Rodents, with one exception, 

 have four molars on each side in both jaws; the fibula distinct throughout life; the upper lip 

 ft; the muffle clad with a velvety coat of fine hairs; and the nostrils either S-shaped or 

 The zygomatic arch is stout, and the malar process does not advance far forward, nor is it 

 supported below by the maxillary process. 



* See Note on p. 83. 



106 



TEETH OF THE TAGUAN. 



