EVEN-TOED UNGULATES. 605 
mule are infertile hybrids of the horse and ass (Zguus 
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 
34 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, t.e., the paunch, 
the reticulwm or honeycomb, so named from the polygonal 
cells on its interior, the psaléertwm 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. (Huxley.) 
