RODENTIA. 589 



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 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 : — 



Anomaluridss, 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). 

 Castoridae — the beaver (Castor), 

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

 other families : — 



Myoxidse — the dormice (Myoxus, etc.') 



Muridee — e.g., the brown rat {Mus decumanus), the 

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

 the wood-mouse (M. syhaticus), the harvest-mouse 

 [M. minutus), the water-voles [Arvicola), the American 

 musk-rat [Fiber zihethicus), the lemming (Myodes), the 

 hamsters [Cricetus). 

 Geomyidse — e.g., the American pouched rat (Geomys iur- 



sarius). 

 Dipodidse — e.g., the Jerboas (Dipus, etc.) 

 Porcupine-like (Hystricomorpha), including the following and 

 some other families : — 



Octodontidss — e.g. , the aquatic Coypu [Myopotamus coypu). 

 Hystricidse — e.g., the porcupine [Hystrix, etc.) 

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

 DasyproctidEe— 1?.^., the Agoutis (Dasyprocta), and the paca 



(Cielogenys). 

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

 Capybara (Hydrochoirus), the largest living Rodent — 

 measuring 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. At birth, 

 there are three pairs of upper incisors, tjut the outermost are soon shed. 

 Lagomyidse : — The Picas or tail-less hares (LagDmys), guinea- 

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

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

 LeporidiE: — e.g., the common hare (Lepus timidus), the 

 mountain hare (Z. variabilis), the rabbit {L. cuniculus). 



