OP THE CLASS MAMMALIA. 29 



pair. If tlie species be horned, the horns form one pair or two 

 pairs ; they are never developed singly, of symmetrical form, from 

 the median line. The post-tympanic does not project downward 

 distinctly from the mastoid, nor supersede it in any Artiodactyle ; 

 and the paroccipital always exceeds both those processes in length. 

 The bony palate extends further back than in the Perissodactyles ; 

 the hinder aperture of the nasal passages is more vertical and com- 

 mences posterior to the last molar tooth. The base of the ptery- 

 goid process is not perforated by the ectocarotid artery. The 

 crowns of the premolars are smaller and less complex than those 

 of the true molars, usually representing 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 Ruminants ; 

 the comparatively small and simple caecum and the spirally folded 

 colon in all Artiodactyles, which equally indicate the mutual affini- 

 ties of the even-toed hoofed quadrupeds, and their claims to be re- 

 garded as a natural group of the Ungulata. The placenta is dif- 

 fused in the Camel-tribe and non-ruminants ; is cotyledonal in the 

 true Ruminants. Many extinct genera, e.g. Chceropotcwms, An- 

 thracotherium, Uyopotamus, Entelodon, DicJwdon, Merycopotamus, 

 Xipltodon, DicJiobime, AnoplotTierium, Microtlieriwn, &c, have been 

 discovered, which once linked together the now broken series of 

 Artiodactyles, represented by the existing genera, Hippopotamus, 

 Sus, DicotyJes, Camelus, Auc7ie?iia, Moschus, CameJopardalis, Cer- 

 vus, Antilope, Ovis, and Bos. 



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

 dinate group of the Artiodactyles, is called Ruminantia, in refer- 

 ence to the second mastication to which the food is subject after 

 having been swallowed ; the act of rumination requiring a pecu- 

 liarly complicated form of stomach. The Ruminants have the 

 ■ cloven foot,' i. e. two hoofed digits on each foot forming a sym- 

 metrical pair, as by the cleavage of a single hoof; in most species 

 two small supplementary hoofed toes are added. The metacarpals 

 of the two functional 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 Rumi- 

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

 canines are contiguous, and, save in the Camel-tribe, similar to the 

 six lower incisors, forming part of the same terminal series of 

 eight teeth, between which and the molar series there is a wide 



