IV. VERTEBRATA: MAMMALIA, UNGULATA. 639 



large opening in front of the orbit in which a part of the masseter 

 muscle is attached. 



The rodents are distinguished from the ungulates, which, like 

 them, are herbivorous, by the usually smaller size, the possession 

 of claws, five toes (sometimes reduced to three), the occurrence 

 usually of a clavicle, and a discoid placenta. The mammae are 

 inguinal in position and, corresponding to the great fertility, are 

 very numerous. The occurrence of glands with a strong-smelling 

 secretion, which open near the anus, is common. About nine 

 hundred living species are known, occurring in all regions except 

 the Australian. The order appears in the eocene. 



Sub Order I. DUPLICIDENTATA (Lagomorpha), two upper incisors, 

 includes the hares, Lepus,* and the picas, Lagomys.* 



Sub Order II. SCIUROMORPHA. The squirrels, SCIURID.E, are distin- 

 guished by the soft fur and bushy tail. Sciurus,* squirrels ; Cynomys,* 

 prairie dogs ; Sciuropterus,* flying squirrels. The CASTORID^E have soft fur 

 and scaly tail. Castor fiber * beaver of Europe and America. 



Sub Order III. MYOMORPHA, rats and mice. Mus musculus,* 

 common mouse; Mus rattus,* house rat, once abundant but now replaced 

 by the gray rat, M. decumanus,* an immigrant from Asia. White rats are 

 albinos of M. rattus. Fiber zibethicus* musk rat; Arvieola* field mice. 



Sub Order III. HYSTRICOMORPHA. The porcupines (HYSTRICID^E) 

 have spines; the Old World forms, Hystrix, are terrestrial, ours (Erethyzon) 

 arboreal. The CAVHD.E of South America have hoof-like claws. Cavia 

 cobaya, guinea pig. Hydrochosrtis, capybara, the largest existing rodent. 



Order V. Ungulata. 



Under the heading of Ungulata, or hoofed animals, are here 

 included two groups of living animals in which the body weight 

 is supported on hoofs on the tips of the toes, and which are sharply 

 marked off from other forms. If, however, the fossils are in- 

 cluded, the limits of the group must be extended so that it includes 

 the elephants and conies of the existing fauna as well as several 

 extinct forms, for these so interlock and intergrade that sharp 

 lines cannot be drawn. 



The ungulates, which arise from common ancestors, the Oon- 

 dylarthra, the representatives of which occur in the eocene of 

 America (Phenacodon), are preeminently herbivorous; the canines 

 are rarely well developed, the molars numerous and adapted to 

 grinding the food, more or less flattened and frequently with 

 folded enamel. The mammae are inguinal, the uterus bicornuate, 

 and the placenta either diffuse or (most ruminants) cotyledonary 

 (fig. 659). The legs are exclusively locomotor structures and, to 



