564 CHORDATA. 
This peculiar dentition is not confined to the Rodentia ; two persistent 
permanently-sharp incisors of a large size, with corresponding reduction 
or loss of the others, appear to have been evolved in several independent 
series of Mammalia. In present-day forms, the wombat (Fig. 349) 
amongst Diprotodontia, the aye-aye (Fig. 394) amongst the lemurs, and 
Hyrax (Hyracoidea) of the Ungulata (Fig. 386), all have essentially 
the same adaptation, whilst the single pair of persistent incisors of the 
elephants may also be recalled. 
In extinct types, the important orders of 77//odontia and Typotheria 
have a somewhat similar arrangement, the former being often regarded 
as transition types between Rodentia, Carnivora and Ungulata. 
All Rodentia are herbivorous and usually have a long 
intestine and large czecum. 
The brain is of a low type, proportionately small; the 
cerebrum is little convoluted and too small to reach back- 
wards over the cerebellum. 
The uterus is often double, as in the rabbit, or is widely 
bicornuate, and there is usually a high fecundity. The 
placenta is discoidal and deciduate. 
From these and other characters the Rodentia occupy a 
low place amongst utheria, but apparently their adapted 
dentition has enabled them to become the most widely 
distributed and abundant mammalian order. ‘Their present 
day headquarters appear to be the Neogcean realm 
(South America) in which there occur enormous numbers, 
including the Capybara or largest existing rodent. 
The order can be traced back to the Upper Eocene, 
below which it is more or less merged into the earliest 
Ungulata. 
The hares, rabbits (Zeporide) and the picas (Lagomyida) 
are placed in a sub-order, Duplicidentata, characterised by 
more or less enamel on the inner surface of the incisors, the 
presence of a small inner pair of incisors in the upper jaw, 
a tendency to a larger number of molars and the descent 
of the testes into a scrotal sac. They are confined to the 
Arctogcean realm. 
The rest of the Rodentia form the large sub-order Sim- 
plicidentata, with only one pair of upper incisors, having 
enamel only on their outer surfaces; the molar teeth 
tend to become reduced in number and the testes are 
mainly abdominal. They are of world-wide distribution 
and include the /ystricomorpha or porcupine-like forms 
