. The Cambridge natural history. Zoology. CHARACTERS OF RODENTS 459 Some, however, such as the A'oles, are aquatic; others, the Squirrels, are arboreal, and there are " flying " Eodents exemplified by the genus Anomalurus. 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 incisnrs and the molars. Another peculiarity is, that in many cases the de
. The Cambridge natural history. Zoology. CHARACTERS OF RODENTS 459 Some, however, such as the A'oles, are aquatic; others, the Squirrels, are arboreal, and there are " flying " Eodents exemplified by the genus Anomalurus. 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 incisnrs and the molars. Another peculiarity is, that in many cases the dentition is absolutely monophyodont. In such forms as the Muridae there seems to be no milk de^jtition pjirr, 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 jig. 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 found that in Hydro- choerus} 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 the 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 Froc. Zool. Soc. 1884, p. Side view of skull of Cape Jumping Hare (Pei/eh-s caffer). x 4. ^1-S, Alisphenoid ; , exoccipital; Fr, frontal ; L, laclirymal ; jl/f, malar ; M.
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