MAMMALIA 



467 



the caecum is relatively small. The pigs and hippopotamuses, forming 

 the group Suina, are the least specialised of these animals. In corre- 

 spondence with their habit of dwelling in marshes and forests, where the 

 ground is soft and a broad tread is needed, they have four well-developed 

 digits on each foot, though the middle two alone touch the ground, and their 

 metacarpal and metatarsal bones are not fused into " cannon bones. " The 



dental formula of the pie is 2? — lAlj. The canines are large, grow 



3. 1, 4, 3 

 throughout life like the incisors of the rabbit, and in the male form 

 tusks ; the grinding teeth are knobbed, not ridged, and the stomach has 

 not the complicated form of that of animals that chew the cud. Cattle, 

 with deer, giraffes, antelopes, and sheep, form the group Ruminantia. 

 Here only the third and fourth digits are complete, and the fused 



Fig. 345. — A side view of a horse's skull, roots of teeth exposed. 

 — From Thomson. 



P., Parietal ; F. t frontal ; «., nasal ; pin,, premaxilla ; w., maxilla ; /. jugal ; 

 /., lachrymal; sq., squamosal;//., paroccipital process; c, canine or 

 "tush" ; C., condyle. 



metacarpals and metatarsals of these digits form "cannon bones." 1 

 The fibula is represented only by a small nodule of bone attached to 

 the distal end of the tibia. There are no incisors or canines in the 



upper jaw, the dental formula being °' °' 3' 3 . The ridges of the grind- 



3» I > 3» 3 

 ing teeth are crescentic and run fore and aft along the jaw. Such 

 teeth are called selenodont. The animals "chew the cud," and 

 in connection with this habit have a complicated stomach, with 

 four compartments, shown in Fig. 342. The food when it is first 

 swallowed passes into the rumen or paunch at the left-hand end of the 

 organ, the walls of which are beset with small processes or villi. Here 

 it is kept till the animal is ready to chew it, becoming meanwhile 

 somewhat softened. It is then passed back into the mouth, chewed 



Vestiges of the second and fifth digits are found in deer, 



