EVEN-TOED UNGULATES. 605 



mule are infertile hybrids of the horse and ass (Equus 

 asinus Linn.). 



Artiodactyles. The even-toed Ungulates comprise the 

 peccary, pig, hippopotamus, and the Ruminants represented 

 by the deer, sheep, ox, and camel. The pig and peccary are the 

 descendants of a number of extinct earlier forms which flour- 

 ished in the Tertiary Period ; the pig, as Marsh observes, 

 having held its own with characteristic pertinacity. The 

 Hippopotamus (Fig. 522) has a large head, with large canines, 

 a clumsy body, and short, four-toed legs. Hippopotamus 

 amphibius Linn., ranges from the Upper Nile to the Cape of 

 Good Hope, and westward to Senegambia. It is nearly 

 3 metres (11 feet) in length. 



Ruminantia. The remaining Artiodactyles are called 

 Ruminants, from the fact that they chew their cud. The 

 molars are provided with two double crescent-shaped folds 

 (compare Fig. 490). The stomach (Fig. 523) is divided into 

 at least three, usually four compartments, i.e., the paunch, 

 the reticulum or honeycomb, so named from the polygonal 

 cells on its interior, the psalterium or manyplies, and lastly 

 the rennet or true stomach. When a sheep, cow, or any 

 other Ruminant feeds, it thrusts out its long tongue, seizes 

 a bunch of grass, and bites it off by pressing the incisors 

 of the lower jaw against the toothless gum of the opposing 

 part of the upper jaw ; the mouthful of grass is then swal- 

 lowed, mixed with much saliva. When its appetite is satis- 

 fied it seeks a retired spot away from its carnivorous ene- 

 mies, if not a domesticated animal, and after lying down, 

 suddenly regurgitates a ball of grass, the cud,* which it slow- 

 ly grinds up between its molar teeth into a pulp. The 

 cropped grass passes into the honeycomb and paunch ; the 

 manyplies serves as a strainer for the pulp, which in the 

 fourth stomach is digested by the gastric juice. 



Among a number of fossil forms leading up to the exist- 



* The regurgitation of the cud is probably due to a sudden and sim- 

 ultaneous contraction of the diaphragm and of the abdominal muscles, 

 which compresses the contents of the rumen and reticulum, and 

 drives the sodden fodder against the cardiac aperture of the stomach, 

 which opens and the cud is propelled into the mouth, f Huxley.) 



