772 MAMMALIA. 
rapidly. The incisors are rootless, growing from persistent 
pulps, and the same is sometimes true of the bunodont or 
lophodont back teeth. There is never more than a pair of 
lower incisors, and in most cases the upper jaw has only a 
pair. There are no canines, and the skin projects as a 
hairy pad into the mouth through the gap between incisors 
and premolars. 
The feet are plantigrade or semi-plantigrade, generally with 
five clawed or slightly hoofed digits. Clavicles, though often 
rudimentary, are generally present. The scapula has usually 
a long acromion process, sometimes with a metacromion. 
The condyle of the mandible (and the corresponding 
articular surface for it) is usually elongated, and the jaw 
moves backward and forward. The mandible has an 
abruptly narrowed and rounded symphysis, and a very large 
angular portion. The orbits are confluent with the 
temporal fosse. The zygomatic arch is complete, but the 
jugal is restricted to the middle of it. The premaxille are 
large, the palatines small. There is generally a distinct 
interparietal bone. The tympanic bulle are always de- 
teloped, and often large. 
The cerebral hemispheres are almost without convolu- 
vions, and leave the cerebellum uncovered. The skin is 
generally thin, and the panniculus carnosus but slightly 
developed. The intestine has a large caecum, except in 
Myoxide. Special anal or perineal or other glands secreting 
odoriferous substances are frequent. 
The testes are inguinal or abdominal; only in the hares 
and rabbits do they completely descend into scrotal sacs. 
The mamme are on the abdomen, or on the abdomen 
and thorax. The uterus is double or very markedly 
bicornuate. There is a provisional yolk-sac placenta; the 
allantoic placenta is discoidal and deciduate. 
The Rodents are very widely distributed, but are most abundant in 
S. America, where they form a very characteristic part of the fauna. 
Out of seventeen existing families, nine are represented there, and four 
are peculiar to it. 
The Rodents are divided into four sub-orders :— 
1. Sciuromorpha.—Squirrels (Scéurus), marmots (Arctomys),- 
prairie-dogs (Cynomys), and beavers (Castor). P 
z. Myomorpha.—Rats and mice (4/us), voles (Arvzcola), lemmings 
(Myodes), and jerboas (Dipus). 
