564 CHORD AT A, 



This peculiar dentition is not confined to the Rodeiitia; 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 {Hyi^acoidea) 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 Tillodontia 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 caecum. 



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 Eutheria, 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 Neogoean 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 {Leporidce) and the picas {Lagomyidce) 

 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 

 Arctogoean 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 Hystricomorpha or porcupine-like forms 



