IV. VERTEBRATA: MAMMALIA, UNO UL AT A. 641 



how the other digits disappear, digit I being lost still earlier. 

 Since the weight of the body rests more upon the hind legs than 

 upon the front ones, the former are the first to become modified. 

 Since we are able, by using abundant paleontological material, to 

 follow in detail the lines of descent of both artiodactyles and peris- 

 sodactyles, the conclusion is certain that these form diverging 

 series, distinct from the beginning. In each series most of the 

 common characters enumerated above have been independently 

 acquired so that the uniformity in appearance of the various 

 groups of ungulates is in great part the result of convergence. 

 The discussion of the fossils will be given under a separate head. 



Sub Order I. PERISSODACTYLA (Solidungula). The dentition is 

 peculiar in having molars and premolars (with more or less pronounced 

 enamel folds) of equal size; a second character is the predominant devel- 

 opment of the middle toe, the others in the three existing families reduced 

 to different degrees. TAPIRID^E, fore feet four-toed, hind feet three-toed; 

 teeth i^||; nose elongate into a proboscis. Tapirus, tapirs, tropical Amer- 

 ica and India. RHINOCEROTIDJE, three toes on all feet, teeth ff; one or 

 two horns on the nasal bones, these without skeleton; skin thick, hairless, 

 hence these were formerly united with elephants as Pachydermata. 

 Rhinoceros, a single horn, India; Ceratorhmus (Asia), Atelodus (Africa), 

 have two horns. EQUID.E, a single functional toe, toes II and IV forming 

 splint bones (fig. 664, c); teeth |{|| ; Equus cdballus* horse, a native of 

 Asia; E. asinus, ass; E. zebra. Hybrids between jackass and mare are 

 called mules; between stallion and she-ass, hinnies. 



Sub Order II. ARTIODACTYLA. Besides the features of the feet, 

 these forms have the premolars, three or four in number, smaller than the 

 molars. The species are much more numerous than the perissodactyles 

 and may be divided into three sections. Section I, NON-RUMINANTIA 

 (Bunodontia); omnivorous and have correspondingly a bunodont dentition, 



Jj|, the canines frequently developed into tusks; the stomach is 



usually simple, but is occasionally divided into three chambers (Dicotyles, 

 Hippopotamus), although rumination does not occur. The leg skeleton is 

 little modified (fig. 664, D), ulna and fibula not being reduced, and meta- 

 carpals and metatarsals separate. HIPPOPOTAMUS ; all four toes reach the 

 ground; skin thick (' pachyderm'), body heavy; living species all African. 

 Hippopotamus. SUID^E; two functional toes, skin with bristles, snout 

 proboscis-like. Sus scrofa, swine ; Dicotyles* peccaries of warmer 

 America. 



Section II. RUMINANTIA (Pecora); teeth and stomach are adapted to 

 the exclusively herbivorous diet. The stomach (fig. 665) is divided into 

 two portions, each again subdivided. The first of these, the rumen, or 

 paunch (ru), receives the food as it is eaten; then at a time of quiet it is 

 regurgitated into the mouth and ground by the molars (' chewing the cud '). 

 It then passes back, this time into the second division, the honeycomb, or 



