CHARACTERS OF RODENTS 



459 



Some, however, such as the Voles, are aquatic ; others, e.g. the 

 Squirrels, are arboreal, and there are " flying " Eodents exemplified 

 by the genus Anomalurvs. Their range of habitat is in fact as 

 wide as that of any other Order of mammals, and wider than 

 that of most. 



The most distinct anatomical characteristic of the Eodents 

 concerns the teeth. They are without exception entirely deprived 

 of canines. Thus there is a long diastema between the incisors 

 and the molars. Another peculiarity is, that in many cases the 

 dentition is absolutely 

 monophyodont. In 

 such forms as the 



Srf^ ^ '^-^^ 



AxO 



\osiyAs -p^ 



F^rx: 



Side view of .slvuU of Cape Jnmpiiig Hare 

 (Pedetes caffer). x f. Aii, Alisphenoid ; Ex.O, 

 exoccipital ; Fr, frontal ; Z, lachrymal ; Mu, 

 malar ; M.v, maxilla ; Na, nasal ; OS, orbito- 

 sphenoid ; Pa, parietal ; Per points to the large 

 siipratympanic or mastoid bulla ; PMx, \n-ii- 

 niaxilla ; Sq, squamosal ; Tij, tympanic, (From 

 Flower's Osteology.) 



Muridae there seems to 

 be no milk dentition 

 at all. In that family 

 there are only three 

 molars ; but in other 

 types where there are 

 four, five, or six molars, 

 the first one, two, or 

 three, as the case may fig. 231 

 be, have milk pre- 

 decessors, and may thus 

 be termed premolars. 

 This has been definitely 

 proved to be the case 

 in the common Eabbit, which has the unusually large number of six 

 grinding teeth in each half of the upper jaw when adult. The 

 first three of these have milk forerunners. On the other hand the 

 existence of four molars does not apparently always argue that 

 the first is a premolar ; for Sir W. Flower fomid that in Hydro- 

 choertis} none of the teeth had any forerunners, at any rate so 

 far as could be detected from the examination of a very young 

 animal. The Eabbit appears to be also exceptional, in that the 

 second incisor of tlie upper jaw and the incisor of the lower jaw 

 have milk forerunners. In any case the tendency towards mono- 

 phyodontism is peculiarly well-marked in this group of mammals. 

 The incisors of Eodents are as a rule in each jaw a single pair of 

 long and strong teeth, which grow from persistent pulps, and 

 1 Proc. Zool. Soc. 1884, ].. 252. 



