xi THE ORDER UNGULATA 143 



tinctions in the limbs, there are so many others correlated with 

 them in the number of the vertebrae, the structure of the cranial 

 bones, of the teeth, of the digestive organs, etc., that there can 

 be no question about their forming natural divisions, very 

 important to palaeontologists, as it often happens that the 

 position of an extinct and little-known form can be determined 

 from a very small fragment of bone. 



Each of these groups is further divided into genera, the 

 names of which in what appears, in the present state of 

 knowledge, to be their natural position and relation to each 

 other are indicated on the diagram. From this it will be 

 seen that the existing Perissodactyles (excluding Hyrax, the 

 position of which is doubtful, though I am inclined to consider 

 it as an aberrant member of this group) consist of three 

 groups, the tapirs, the rhinoceroses, and the horses, each 

 represented by but few species, and (except in the case of 

 the horse, through the agency of man) of rather restricted 

 geographical distribution. 



These groups at present are separated by very decided 

 intervals, so much so, that one of them, containing the horses, 

 has been considered by many naturalists as forming an order 

 apart, the Solidungula. 



The existing Artiodactyles range themselves around two 

 principal types, the tubercular-toothed, or bunodont, and the 

 crescentic-toothed, or selenodont. To the former belong the 

 pigs in all their modifications, including the babirussa and 

 wart-hogs, and the hippopotamus and peccary. To the latter 

 the ruminants, i.e. vast numbers of species of animals included 

 under the general designation of sheep, oxen, goats, antelopes, 

 deer, musks, giraffes, and the two allied though aberrant 

 forms, the camels, and the Tragulidce or chevrotains, an 

 interesting little group long confounded with the musk deer. 



The two extremes of this division, represented by the pigs 

 and the hollow-horned ruminants, seem to have very little in 

 common at first sight, and if we were acquainted with the 

 organisation only of the existing species, we might be justified 

 in treating them as belonging to very distinct groups. But 

 even among existing forms there are some examples, which may 

 almost be called intercalary types, so widely do they depart 



