FAMILIES OF ARTIODACTYLA. 577 



There are primitive extinct Artiodactyla which connect 

 the four modern groups — Suina, Tylopoda, Tragulina, and 

 Pecora. Thus they unite the bunodont types, such as pigs, 

 with cone-like tubercles on the crowns of the molars, and 

 the selenodont types, such as cattle, with the tubercles 

 expanded from before backwards and curved in crescents. 



Group I. Suina — Hippopotamus, Pigs, and Peccaries. The molars 

 are bunodont ; the third and fourth metacarpals and metatarsals are 

 not completely fused as " cannon-bones." 



HippopotamidfE : — ^huge African mammals, included in the single genus 

 Hippopotamus. They spend the day in the rivers and lakes, 

 swimming and diving well, but usually remaining concealed. At 

 night they come on land and browse on grass and herbage. The 

 skin is extremely thick, with a few hairs restricted to the snout, 

 head, neck, and tail. There are four toes on each foot, all 

 reaching the ground. The rootless incisors continue growing ; 



so do the large curved canines ; the dental formula is — ^^1?- 



i-3i 143' 

 The stomach has three chambers ; there is no caecum. 

 SuidK : — the Old World boars and pigs, characterised by the mobile 

 snout and terminal nostrils. There are four well-developed digits 

 on the narrow feet, but the second and fifth do not reach the 

 ground in walking. The incisors are rooted, the upper canine 

 curves outwards or upwards. The stomach is almost simple, but 

 has more or less of a cardiac pouch ; there is a caecum. 

 Examples : — Sus, W^ ; Babirusa -H-Ifj the male with remarkable 

 canines, the upper pair growing upwards from their base 

 through the skin, arching backwards as far as the forehead, and 

 sometimes forwards and downwards again, the lower pair with 

 a more or less parallel course ; Phacochcerus, the wart-hog. 

 Dicotylidae : — the New World Peccaries (Dicotyles), with a snout like 

 that of pigs, with four toes on the fore-feet, and three behind. 

 The incisors are rooted, the upper canines are directed down- 

 wards, the dental formula is JHf- The stomach is complex, and 

 there is a caecum. 

 Group 2. — Tylopoda, comprising the family CamelidiE — the Camels of 

 the Old World and the Llamas of S. America. The limbs are 

 long, with only the third and fourth digits developed ; the two meta- 

 carpals and metatarsals are united for the greater part of their 

 length, but there is a deep distal cleft ; the tips of the digits have 

 very incomplete hoofs, and the animals walk on a broad pad 

 of skin surrounding the middle phalanges. The femur is long 

 and vertical, and the knee is low down. Of the three upper 

 incisors only one persists in adult life, as an isolated sharp tooth, 

 those of the lower jaw are long and slope forwards. There are 

 canines both above and below. The molars are selenodont. The 

 camels and llamas ruminate, and the stomach is divided into 

 three chambers, of which the first two have remarkable pouches 

 2 O 



