RODENTIA. 749 



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 fossae. The zygomatic arch is complete, but the 

 jugal is restricted to the middle of it. The premaxillae are 

 large, the palatines small. There is generally a distinct 

 interparietal bone. The tympanic bullae are always de- 

 veloped, and often large. 



The cerebral hemispheres are almost without convolu- 

 tions, 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 

 Myoxidae. 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 mammae 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 (Scinnis), marmots (Arcto/nys], 



prairie-dogs (Cynomys), and beavers (Castor}. 



2. Myomorpha. Rats and mice (ASus), voles (Arvtcola), lemmings 



(iWyodes), and jerboas (Dipits}. 



3. Hystricomorpha. Porcupines (Hystrix), agoutis (Dasyprocta), 



guinea-pigs (Cavia), and the S. American capybara 

 (Hydrochcerus), the largest living Rodent, measuring about 

 4 ft. in length. 



