714 MAMMALIA. 



usually a long acromion process, sometimes with a meta- 

 cromion. 



The condyle of the mandible is elongated from before 

 backwards, and the jaw moves backward and forward 

 (unimpeded by any postglenoid process of the squamosal). 

 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 developed, 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 mammas 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 affinities are not well known. A relation to the elephants has 

 been suggested by various authorities. The Typotheria, an extinct 

 S. American sub-order of Ungulates, resemble Rodents in many 

 respects ; but, according to Lydekker, this is merely an example of con- 

 vergence. Another possibly related sub-order is that of the Tillodontia. 



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 (Sciurus), marmots {Arctomys), 



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



2. Myomorpha. — Rats and mice {Mus), voles (Arvicola), lemmings 



(Myodes), and jerboas (Dipus). 



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. 



4. Lagomorpha. — Hares and rabbits (Lepus), and the picas or 



tailless hares (Lagomys). 



