xi THE EVEN- TO ED UNGULATES 149 



traced with few breaks, through the successively modified 

 miocene genera Ch&rotherium and Palceochcerus to the genus 

 Sus, or true pig, in which the dentition undergoes some 

 remarkable specialisations, as the upturning of the upper 

 canines, and great development and extremely tuberculated 

 character of the posterior molars, which are both singularly 

 exaggerated in some modern offsets of the pig family, the first 

 in the babirussa, and the second in the wart-hog (Phacochczrus). 

 More distantly related to the true pigs are the hippopotamus 

 on the one hand, and the peccary on the other. In relation 

 to the first, not found anteriorly to the latest miocene, it is 

 significant that the earliest known forms had the more 

 generalised number of incisor teeth (six) instead of four as 

 in the modern hippopotamus, and hence has been made into" a 

 genus by itself, called Hexaprotodon. 



The researches of Leidy into the ancient (miocene and 

 early pliocene) fauna of Nebraska have furnished evidence of 

 a remarkable group of animals now entirely extinct, the 

 Oreodontidce, the characters of which are perfectly inter- 

 mediate between those of the pigs and the ruminants : 

 animals with pig -like feet and complete number of incisors, 

 canines, and molars, but with the latter important set of 

 teeth, formed precisely on the same type as those of the 

 deer. Within this particular group Leidy has noted a 

 curious series of slight modifications coinciding with the 

 successive age of thei strata, in which the remains were found. 

 Agriochcerus, the most ancient, approaches nearer to Chcero- 

 potamus, has orbits open behind and very shallow-crowned 

 teeth. Then follows Oreodon proper, and lastly Merycfiyus. 

 more like the modern ruminants. 



To return to the European forms, in the genus Gelocus, 

 where the union of the two principal bones of the metapodium 

 first occurs, Kowalevsky has noticed the gradual way in 

 which this change seems to have been brought about in 

 successive epochs of eocene and early miocene strata, at first 

 free in the young, and only coalescing in old animals, afterwards 

 coalescing at a much earlier age. The gradual perfecting of 

 the foot by the development of the ridge round the lower 

 articular end of the metapodium in later forms, the ridge 



