570 CHORDATA 



tooth structure. The sin!,'Ie-toed horses appeared in the pleistocene with 

 Fliohippiis (s) and then Eqiius itself (o). It is a jieeullar fact that the horse 

 entirely died' out in America, althoui,di the chief part of its history was enacted 

 here. The A:MBLYP0DA, mentioned above, were semi-plantigrade penta- 

 dactvle forms, appearing in the lowest eocene, and reaching, in Vintalhcnum 

 (Dhwccnis) an elephantine size. The TOXODOXTLV of the South American 

 tcrtiaries combined perissodactvle, rodent, hyracoid, and proboscidian features, 

 while the TILLODOXTIA of the eocene recall both carnivores and rodents. 



Order VII. Proboscidia. 



The elephants and their allies, with their hoofs and herbivorous dentition 

 and lack of clavicle, are closely related to the ungulates. They are characterized 

 bv their thick skin ('pachyderm'), the large, massive, five-toed legs, and espe- 

 cially by the nose drawn' out into a long proboscis, lastly by the ^ dentition. 



Canines are lacking, but the incisors of the 

 upper jaw continue to grow throughout 

 life, forming the well-known tusks. In 

 the living elephajits there are but a single 

 pair of tusks, but in some extinct Mas- 

 lodaiis there were a second smaller pair in 

 the lower jaw, while Dinothcriiim had 

 only the lower incisors, these projecting 

 downwards. The molars (in Mastodon 



^ , o T -I fi ffi,.,, •,„. and Dinothcriiim with normal replace- 

 FlO. bib. — Inside ot Icitlouer )a\\ . . ' 



of Elcpl.as i„dicus, the alveoli opened ment and cusps) consist of numerous 

 (after Owen), i, functional molar; 2, plates of enamel and dentine united by 

 its successor. cement, and undergo a horizontal dis- 



placement. Of the three large molars 

 and premolars only one at a time is functional (fig. 618, i); when worn out the 

 next one behind (2) takes its place. Further features are a uterus bicornis, a 

 zonary placenta, and two pectoral mamma;. 



Eleph.^ntid.e: Elcplias iiidiciis, small ears; E. africaiins, large ears. E. 

 priinigcniiis, mammoth, pleistocene; specimens found frozen in ice in Siberia 

 have close woolly hair, in some places three feet long. Mastodon, with tuber- 

 culate teeth, miocene and pliocene. Dinotherid.e, Dinolhcrium, Old World 

 miocene. 



Order VIIL Hyracoidea. 



The single genus Hyrax, including species from western Asia and Africa, 

 with four-toed front feet, hind feet with three toes, the digits with nails, the 

 placenta zonary, and the dentition -i":\ 5, forms this grouji, no fossils being known. 

 Hyrax syriacus is sujipiosed to be the 'coney' of the Bible. 



Order IX. Sirenia. 



This order consists of a few aquatic whale-like mammals, with the fore 

 limbs fin-like, hind legs lacking, and a horizontal caudal fin. They live in 

 shallow seas or in the mouths of rivers, where they feed on the vegetation, which 

 they chew with jaws covered with horny ]ilates. The teeth (in the fossil Pro- 

 rostonnis illji) are reduced or entirely lacking. The fore legs are pentadactyle, 

 often have rudimentary nails and always a flexible elbow. The two pectoral 

 mamma; are possibly the germ of truth in the mermaid myth. Manatns aincri- 

 canus* manatee, six cervical vertebnc, eight to ten molars; Halicore diigo)ig, 

 Indo-Pacific; Rliytina stcllcri, northern Pacific, exterminated in 1768. 



