41 



less complex than those of the true molars, usually represent- 

 ing half of such crown. The last milk-molar is trilobed. 



To these osteological and dental characters may be added 

 some important modifications of internal structure, as, e. g., the 

 complex form of the stomach in the Hippopotamus, Peccari, 

 and Euminants ; the comparatively small and simple caecum 

 and the spirally folded colon in all Artiodactyles, which equally 

 indicate the mutual affinities of the even-toed hoofed quad- 

 rupeds, and their claims to be regarded as a natural group of 

 the UNGULATA. The placenta is diffused in the Camel-tribe 

 and non-ruminants ; is cotyledonal in the true Ruminants. 

 Many extinct genera, e. g. Chceropotamus., AntJiracotherium, 

 Hyopotamus, Entelodon, Dichodon, Merycopotamus, Xiphodon, 

 Dichobune, Anoplotherium, Microtherium, &c., have been dis- 

 covered, which once linked together the now broken series 

 of Artiodactyles, represented by the existing genera, Hippo- 

 potamus, Sus, DicotyleS) Camelus, Auchenia, Moschus, Camelo- 

 pardatis, Cervus, Antilope, Ovis, and Bos. 



A well-marked, and at the present day very extensive 

 subordinate group of the Artiodactyles, is called Ruminantia, 

 in reference to the second mastication to which the food is 

 subject after having been swallowed ; the act of rumination 

 requiring a peculiarly complicated form of stomach. The 

 Ruminants have the ' cloven foot,' i. e. two hoofed digits on 

 each foot forming a symmetrical pair, as by the cleavage of a 

 single hoof ; in most species there is added a pair of small 

 supplementary hoofed toes. The metacarpals of the two func- 

 tional toes coalesce to form a single cannon-bone,' as do the 

 corresponding metatarsals. The Camel-tribe have the upper 

 incisors reduced to a single pair ; in the rest of the Ruminants 

 the upper incisors are replaced by a callous pad. The lower 

 canines are contiguous to the six lower incisors, and, save in 

 the Camel-tribe, are similar to them, forming part of the same 

 terminal series of eight teeth, between which and the molar 

 series there is a wide interval. The true molars have their 

 grinding surface marked by two double crescents, the con- 



