8 CLASSIFICATION OF THE MAMMALIA. [CHAP. 



species of elephants, constituting the group called Proboscidea, 

 These, however, are now known to be the survivors of a large 

 series of similar animals Mammoths, Mastodons, and Dino- 

 theria which as we pass backwards in time gradually assume 

 a more ordinary or generalised type ; and the interval which 

 was lately supposed to exist between even these and the 

 rest of the class is partially bridged over by the discovery 

 in American Eocene and early Miocene formations of the 

 gigantic Dinocerata, evidently offshoots of the great group 

 of hoofed animals or Ungtilata, represented in the actual 

 fauna by the Horses, Rhinoceroses, Tapirs, Swine, and 

 Ruminants. 



Almost as isolated as the Proboscidea among existing 

 Mammals are the few small species constituting the genus 

 Hyrax, and in their case palaeontology affords no help at 

 present, and therefore, pending further discoveries, it has 

 been thought advisable in most recent systems to give them 

 the honour of an order to themselves under the name of 

 Hyracoidea. But the number of extinct forms already 

 known allied to the Ungulata, but not coming under the 

 definition of either of the two groups (Artiodactyla and 

 Perissodactyla) under which all existing species range them- 

 selves, is so great that either many new orders must be 

 made for their reception or the definition of the old order 

 Ungulata so far extended as to receive them all, in which 

 case both Proboscidea and Hyracoidea might be included 

 within it. 



Again, the Rodentia or gnawing animals Rabbits, Rats, 

 Squirrels, Porcupines, Beavers, &c. are, if we look only at 

 the present state of the class, most isolated. No one can 

 doubt what is meant by a rodent animal, or have any 

 difficulty about defining it clearly, at least by its dental 

 characters ; yet our definitions break down before the 



