RODENTIA. 711 



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. 



Sub-order SiMPLlciDENTATA. 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 : 



Anomaluridse the Ethiopian arboreal genus Anomalurus, 

 with a lateral parachute of skin. 



Sciuridse the squirrels (Sciurus), the flying squirrels 

 Pteromys and Sciuropterus with a parachute of skin 

 connecting the fore and hind limbs, the marmots (Arc- 

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

 or sousliks (Spermophilus}. 



Castoridse the beaver ( Castor]. 



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

 families : 



Myoxidse the dormice (Myoxtts, <2rY. ) 



Muridse e.g., the brown rat (Mus decumamts], the black 

 rat (M. rattus], the house mouse ( M. musculus), the wood- 

 mouse (M. sylvaticus], the harvest mouse (M. minutus\ 

 the water-voles (Arvicold], the American musk rat (Fiber 

 zibethicus}, the lemming (Myodes], the hamsters (Cricetus). 



Geomyidoe e.g., the American pouched rat (Geomys bur- 

 sarius}. 



Dipodidae e.g., the Jerboas (Dipiis, &*c.) 



Porcupine-like (Hystricomorpha), including the following and 

 some other families : 



Octodontidae e.g., the aquatic Coypu (Myopotamus coypu}. 



Hystricidse e.g., the porcupine (Hystrix, &c. ) 



Chinchillidse e.g., the squirrel-like Chinchilla. 



Dasyproctidae e.g., the Agoutis (Dasyprocta), and the paca 

 ( Ccelogenys. ) 



Caviidse e.g., the guinea pig (Cavid), and the S. American 

 Capybara (Hydrochcerus], the largest living Rodent, meas- 

 uring about four feet in length. 



Sub-order DUPLICIDENTATA. 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. 



Lagomyidae The Picas or tailless hares (Lagomys), guinea- 

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

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

 Leporidae e.g., the common hare (Lepus timidus], the 

 mountain hare (L. variabilis], the rabbit (L. cuniculus}. 



