570 



CHORDATA 



tooth structure. The single-toed horses appeared in the pleistocene with 

 Pliohippus (5) and then Equus itself (6). It is a peculiar fact that the horse 

 entirely died out in America, although the chief part of its history was enacted 

 The AAIBLYPODA, mentioned above, were semi-plantigrade penta- 

 dactyle forms, appearing in the lowest eocene, and reaching, in U intatherium 

 (Dinocerus) an elephantine size. The TOXODONTIA of the South American 

 tertiaries combined perissodactyle, 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 

 by 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 elephaaits there are but a single 

 pair of tusks, but in some extinct Mas- 

 todons there were a second smaller pair in 

 the lower jaw, while Dinotherium had 

 only the lower incisors, these projecting 

 downwards. The molars (in Mastodon 

 and Dinothcriu-m with normal replace : 

 ment and cusps) consist of numerous 

 plates of enamel and dentine united by 

 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 mammae. 



ELEPHANTID^E: Elephas indicus, small ears; E. africanus, large ear?. E. 

 primigenius, 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, Dinotherium, Old World 



I i<;. 618. Inside of left lower jaw 

 of Elephas indicus, the alveoli opened 

 (after Owen), i, functional molar; 2, 

 its successor. 



miocene. 



Order VIII. 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 |U -j, forms this group, no fossils being known. 

 Hyrax syriacns is supposed 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 plates.' The teeth (in the fossil Pro- 

 rostomus i! \ J }) are reduced or entirely lacking. The fore legs are pentadactyle, 

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

 m a in 11132 are possibly the germ of truth in the mermaid myth. Manatus atneri- 

 canus* maiiaicc, six cervical vertebrae, eight to ten molars; Halicore dugong, 

 [ndo-Pacific; Khytinn strllcri, northern Pacific, exterminated in 1768. 



