566 



CHORDATA 



appear in the eocene in the order CREODONTA, plantigrade forms with 

 slightly differentiated dentition; they resemble marsupials, insectivores, as well 

 as the Condylarthra. True carnivores appear in the upper eocene. 



Order V. Rodentia. 



The rodents unite great similarity in appearance with a characteristic 

 dentition. The canines are absent, the molars separated by a large gap (dias- 

 ti- m,i) from the incisors (fig. 614). The latter are strong, chisel-like, have per- 

 sistent pulps and grow at the root as they are worn away at the cutting edge. 

 Since only the front surface has enamel, wear keeps them constantly sharp. 

 Usually there is but a single incisor; only the Duplicidentata have a second in 

 the upper jaw. The molars are cuspidate or have enamel folds and frequently 

 continue to grow throughout life. Their number is frequently reduced, the 

 formula varying between -f-Jj g and |S J! 5 . Many species have an inflected angle 



of the jaw like that of marsupials. 1 he 

 infraorbital canal is a striking feature in 

 Muridas and Hystricidae (fig. 614, 0), a 

 large opening in front of the orbit for a 

 part of the masseter muscle. 



The rodents are distinguished from 

 the ungulates by the 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 are very numerous. The occur- 

 rence of glands with a strong-smelling 

 secretion, which open near the anus, is 

 common (fig. 603). Over one thousand 

 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, in- 

 cludes hares, Lcpus* and picas, Lagoniys* Sub Order II. SCIUROMOR- 

 PHA. The squirrels, SCIURIOE, have soft fur and bushy tail. Sciurus* 

 squirrels; Cynomys* prairie dogs;Sciuropterus* flying squirrels. The CASTOR- 

 ID, v: have soft fur and scaly tail. Castor fiber* beaver. Sub Order III. MYO- 

 MOKIMIA, rats and mice. Mus nmsculus* common mouse; Mus ratt-us* 

 house rat, once abundant but now replaced by the gray rat, M. decumanns* 

 an immigrant from Asia. White rats are albinos of M. rattus. Fiber zibethicus,* 

 musk rat; An'imla* field mice. Sub Order III. HYSTRICOMORPHA. 

 Tin- porcupines ( HYSTRICID^E) have spines; the Old World forms, Hystrix, are 

 terrestrial, ours (Rrcthyzoii) arboreal. The CAVIID.E, South America, hoof- 

 like claws. Ciivia cobaya, guinea pig. Hydrochcerus, capybara, largest 

 existing rodent. 



Order VI. Ungulata. 



The Ungulata, or hoofed animals, include two groups of living animals in 

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

 sharply marked off from other forms. If, however, the fossils be included, 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 intergrade 

 that sharp lines cannot be drawn. 



Kir,. 614. Skull of porcupine (from 

 Schmarda). /, frontal; im, premaxil- 

 lary; k, temporal fossa continuous in 

 front with orbit; o, infraorbital for- 

 amen, enormous on account of the 

 portion of the masseter muscle which 

 passes through it. 



