CHAP. XV CHARACTERS OF RODENTS 459 
Some, however, such as the Voles, are aquatic; others, e.g. the 
Squirrels, are arboreal, and there are “ flying ” Rodents 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 Rodents 
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 im many cases the 
dentition is absolutely 
monophyodont. In 
such forms as_ the 
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 See 
three, as the case May Fic. 231.—Side view of skull of Cape Jumping Hare 
be, have milk pre- (Pedetes Comes oe a) - saeco Ex.0, 
exoccipital; #7, frontal; JZ, lachrymal; Ma, 
decessors, and may thus malar; Mz, maxilla; Na, nasal; OS, orbito- 
be termed premolars. aaa Pa, parietal ; Per Os J, ne large 
: supratympanic or mastoid bulla; PMz, pre 
This has been definitely maxilla ; Sg, squamosal; 7y, tympanic. (From 
proved to be the case Blowers Oaeclegy.) 
in the common Rabbit, 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 1s 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 Rabbit appears to be also exceptional, in that the 
second inciser 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 Rodents are as a rule in each jaw a single pair of 
long and strong teeth, which gow from persistent pulps, and 
1 Proc. Zool. Soc. 1884, p. 252. 
