MAMMALS. 



391 



ica, the latter region containing the majority of the sub-order to-day. Some 

 of the species, both living and fossil, are giants among the rodents. In 

 the tropical OCTODONTID^ the clavicles are complete, and the molars have 

 internal and external enamel folds; most of the species are terrestrial. 

 Cteuodactylus, from Africa. Octodon, from South America. Myopotamus 

 coypu, of South America, two feet long. HYSTRICID^E ; porcupines ; stout 

 rodents with spines, molars . The species are grouped in two divisions : 

 those of the old world dwell on the ground ; those of the new, climb. 

 Hystrix, with smooth soles and incomplete clavicles, includes the old- 

 world porcupines ; Erethyzon and Synetheres, with tuberculate soles and 

 complete clavicles, those of the new. The latter genus has a prehensile 

 tail. The ChiNCHiLLiD^E, with very soft fur, complete clavicles ; toes five 

 or four in front, four or three behind ; the molars with simple compressed 

 transverse lamellee, are confined to South America. Chinchilla. Lago- 

 stomns includes the burrowing vizcacha. Megamys, of the miocene of 

 Argentina, was as large as, an ox, the largest known rodent. Somewhat 

 closely allied to them was the fossil Castoroides of N. Y. and Ohio pleisto- 

 cene, and the Amblyrhiza from the pleistocene of the West Indies. The 

 CAVIID^E, also South American, have hoof-like claws on the four toes of 

 the fore feet and the three of the hind ; incomplete clavicles, and the 

 molars |- and rootless. Cai'ia contains the guinea-pigs; Hydrochcerus, the 

 capybara, the largest existing rodent. Both these and other genera date 

 from the miocene. The agutis (Dasyprocta) and the paca {C&logcnys) 

 are South American forms with hoof-like claws and semi-rooted molars 

 which form the family DASYPROCTID^E. 



SUB-ORDER 4. LAGOMORPHA (DUPLICIDENTATA). 



Infraorbital canal small ; dentition i | , c g, p \ to f , m ; the enamel 

 of the upper incisors extending on to the sides ; molars high, prismatic, 

 without roots ; tibia and fibula distinct. 



The lagomorphs are readily distinguished by the two pairs of incisors 

 in the upper jaw. The seat of the sub-order is in the northern hem- 

 isphere, but they extend into South America as well. The LEPORID,E, 

 with the premolars f , long ears and incomplete clavicle, includes the hares, 

 and rabbits. Lepus, the principal genus, appears in the miocene of Oregon. 

 About 20 living species. In the LAGOMYID^: the premolars are \ or f , the 

 ears short, and the clavicle complete. The picas (Lagomys} inhabit high 

 altitudes, one species occurring in the Rocky Mountains. The genus 

 appears in the miocene of Bavaria. 



ORDER V. UNGULATA. 



Placental mammals with heterodont, diphyodont dentition ; 

 molars with broad tuberculate or ridged crowns ; clavicles almost 



