/r. VERTEBRATA: 3JAMMALTA, UNGULATA. G39 



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 mamma; are 

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

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

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

 hundred living species are known, occurring in all regions exce[)t 

 the Australian. The order appears in the eocene. 



Sub Order I. DUPLICIDENTATA (La<:;oinori)ha), t\v(j upper incisors, 

 includes the liarcs, Lepiis;* and th(3 picas, Lagomys* 



Sub Order II. SCIUROMORPHA. The squirrels, Sciuiud^, are distin- 

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

 pnuriedogs ; Sciui'opteru.s* flying squirrels. The Castorid.e liave soft fur 

 and scaly tail. Casta rfiher* beaver o£ Europe and America. 



Sub Order III. MYOMOfiPHA, rats and mice. Mus musculas* 

 common mouse; J/h.s rattus* bouse rat, once abundant but now replaced 

 by tlie gray rat, M. decuinanus,'* an immigi-ant from Asia. White rats are 

 albinos of M. rattus. Fiber zihethicus* musk rat; Arvicnla* field mice. 



Sub Order III. HYSTPJCOMORPHA. The porcupines (Hystricid^) 

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

 arboi-cal. The Cavhd.e of Soutli America have hoof-lilie claws. Cavia 

 cobaya, guinea pig. Uydrocharus, 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 intergrado that sharp 

 lines cannot bo drawn. 



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

 dylarthra, the representatives of wliich occur in the eocene of 

 America {Phenacodon), are preeminently herbivorous; the canines 

 are rarely well developed, the molars mtmerous and adapted to 

 grinding the food, more or less flattened aiul frequently with 

 folded enamel. The mammre are inguinal, the uterus bicornnate, 

 and the placenta either diffuse or (most ruminants) cotyledonary 

 (fig. 059). The legs are exclusively loconrotor structures and, to 



