,Q2 MAMMALS. PHYLUM CHORDATA 



upper lip (foreshadowed in the tapirs) and the teeth. There is a 

 single pair of long upper incisors, the tusks, which are used in 

 fighting and digging, and otherwise only a reduced number of 

 cheek teeth, which replace each other throughout life like those 

 of the dogfish. Each is very large, and only one and a half in 

 each quarter of the jaw are in use at any one time. 



ARTIODACTYLA 



These, the ' even-toed ungulates ', are by far the most suc- 

 cessful of the larger herbivores, and include cattle, sheep, deer, 

 pigs, camels, and hippopotamuses. They resemble the perisso- 

 dactyls in habit and general shape, but they have achieved these 

 in different ways. In particular, as the limbs lengthen they do 

 so on an axis running between the third and fourth digits, so that 

 the most highly developed forms, the camels, are unguligrade on 

 the nails of these two only. The first digit is absent from all 

 modern species, and the cattle and deer show various degrees 

 of reduction of digits two and five. In all these, but not in the 

 pigs, the third and fourth metapodials are fused to form a single 

 cannon-bone (Figs. 383, 384). As in the horses, the limbs have 

 pulley-like joints and there is no clavicle. The incisors tend 

 to be reduced, and the cattle and sheep have none in the upper 

 jaw ; the canines may form fighting tusks, or may be small or 

 absent, when they are often functionally replaced by horns or 

 antlers, especially in the males. The cheek teeth generally have 

 a complicated grinding surface, which tends to be selenodont, 

 i.e. to have crescentic ridges, but those of pigs, which are omni- 

 vorous, are bunodont. All the higher forms, including the cattle 

 and deer, have a complicated quadruple division of the stomach 

 (Fig. 385) and chew the cud so that bacteria may assist in the 

 digestion of cellulose. They thus achieve the same result as is 

 got by the refection of rabbits (p. 445). 



The wild boar (Stis scrofa) became extinct in Britain during 

 the later Middle Ages, and the cattle and sheep are represented, 

 apart from domestic breeds, only by a few herds of park cattle 

 (Bos taiirus) which possibly represent enclosed remnants of the 

 former truly wild herds. They are white, with black or red ears. 

 The two existing native British deer are the red [Cervus elaphus) 

 and the roe (Capreolus capreolus). The former was for long 

 confined to the Scottish Highlands, except for small herds in 



