RODENTIA. 711 



The mammse 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. 



Sub-order Simplicidentata. — Rodents with only one pair of upper 

 incisors, with the enamel restricted to the front. 



Squirrel-like (Sciuromorpha), including the following and some 

 other families : — 



Anomalurida; — the Ethiopian arboreal genus Anomaliinis, 

 with a lateral parachute of skin. 



Sciuridse — the squirrels (Sciuriis), the flying squirrels — 

 Pleroinys and Sciiiropterus — with a parachute of skin 

 connecting the fore and hind limbs, the marmots (Ai-c- 

 tomys), the prairie dogs (Cynomys), the pouched marmots 

 or sousliks [Sperviophihis). 



Castoridna — the beaver (Castor). 



Mouse-like (Myomorpha), including the following and some other 

 families : — 



Myoxida; — the dormice (Myoxus, iT'c.) 



Murida; — e.g., the brown rat (Ahis decn77ianits), the black 

 rat {M. rattus), the house mouse (M. musnihis), the wood- 

 mouse (y]/. syhaticus), the harvest mouse (7)/. miiiutzis'), 

 the water-voles (Arvicola), the American musk rat {Fiber 

 zibethictis), the lemming (Myodes), the hamsters (Cricetits). 



Geomyidce — e.g., the American pouched rat (Geoinys bur- 

 sarztts). 



Dipodidfe — e.g., the Jerboas (Dipus, or=c. ) 



Porcupine-like (Hystricomorpha), including the following and 

 some other families : — 



Octodontidte — e.g., the aquatic Coypu {Myopotanius eoypu). 



Hystricida; — e.g., the porcupine (Hystrix, &^r. ) 



ChinchillidEe— e.g., the squirrel-like Chinchilla. 



Dasyproctidje — e.g., the Agoutis (Dasyprocta), and the paca 

 ( Cielogenys. ) 



CaviidK — e.g., the guinea pig (Cavia), and the S. American 

 Capybara (Hydrocha-nis), the largest living Rodent, meas- 

 uring about four feet in length. 



Sub-order Dupijcidentata. — Rodents with two pairs of incisors in 

 the upper jaw, the second pair small and behind the first pair ; the 

 enamel extends to the posterior surfaces, but is thinner there. 



Lagomyidce — The Picas or tailless hares (Lagofuys), guinea- 

 pig-like animals found on the mountains of N. Asia, in 

 S.-E. Europe, and on the Rocky Mountains. 

 LeporidK — e.g., the common hare (Lcpus timidus), the 

 mountain hare (Z. variabilis), the rabbit (Z. cunicubis). 



