ORDERS OF MAMMALIA. 



335 



anatomical peculiarities, •which distinguish them from the Swine of 

 the Old World. 



The Ruminantia form a most natural group of the Ungulata, 

 characterised by the structure of the foot, the dentition, and the 

 structure of the stomach. 



The foot is "cloven," consisting of a symmetrical pair of toes, 

 encased in hoofs, and looking as if produced by the cleavage of a 

 single hoof. The two functional toes (fig. 222) are the third and 

 fourth toes (the middle and ring toes) ; but the second and fifth toes 

 (the index and little toe) may be present in a rudimentary form on 



Fig. 241.— Skull of a hornless Sheep : i Incisors ; c Canines ; m Molars 

 and prgemolars. (After Cuvier.) 



the back of the foot. In no case, however, are these lateral toes so 

 far developed as to touch the ground in walking on a hard surface. 



As regards the dentition, the typical state of things is that there 

 should be no incisor or canine teeth in the upper jaw (fig. 241), but 

 that the lower jaw should have six incisors and two canines, which 

 are all similar in size and form, and constitute a continuous and un- 

 interrupted series of eight teeth placed in the front of the lower jaw. 

 There are six back teeth (prteraolars and molars) on each side of 

 each jaw, and these have grinding surfaces, the enamel ridges of 

 which form double crescents. The typical dental formula, therefore, 

 for a Ruminant is — 



.0—0 



0—0 



' 3—3 ' 1—1 



pm 



3—3. 

 3—3' 



m ' = 32. 



3—3 



