82 THE NEOG^IC REALM. [CHAP. 



one another like those of a wild boar ; and although there were 

 no small teeth in the upper jaw, the lower jaw carried three pairs of 

 spatulate incisors quite unlike those of any other known animal. 

 On the other hand, Homalodontotherium takes its name from having 

 its forty-four teeth arranged in a continuous even series, unbroken 

 either by tusks or by gaps. Whereas the molar teeth of Astrapother- 

 ium present a marked resemblance to those of the rhinoceroses, 

 it is noteworthy that those of the allied genus make a certain 

 approximation to the corresponding teeth of an extinct ungulate 

 from the European Oligocene known as Cadurcotherium, ap- 

 parently more or less closely allied to the rhinoceroses. It is, 

 however, quite improbable that it is to these allied forms we 

 have to look for the origin of the peculiar South American 

 ungulates, seeing that they must have branched off from the primi- 

 tive stock at an earlier stage. 



The third extinct subordinal group, or Toxodontia, takes its 

 name from a gigantic species from the Pampean 



Toxodonts. 



formation to which Owen, on account of the pecu- 

 liarly curved form of the molar teeth, gave the name of Toxodon. 

 Rivalling the largest existing rhinoceros in point of size, the 

 Toxodon has at first sight very much the general appearance of 

 one of those animals, having a massive skull, short limbs and 

 neck, and three-toed feet. The middle toe is, however, scarcely 

 larger than the lateral ones ; and while the bones of the wrist are 

 arranged in the alternating manner, those of the tarsus are placed 

 in a linear series, and the astragalus has a nearly flat, instead of a 

 grooved, superior surface ; the terminal faces of the vertebrae of 

 the neck being also flat, instead of articulating together by ball- 

 and-socket joints. Omitting mention of other more or less strongly 

 pronounced peculiarities in the structure of the limbs, attention 

 may be specially directed to the teeth, which differ from those of 

 all existing ungulates in that the whole of them grow continuously 

 throughout the life of their owner, without ever forming roots. 

 The front or incisor teeth are chisel-like, and thus resemble in 

 form, although not in number, those of a beaver or rabbit; and, 

 indeed, in the general conformation of the teeth, as well as in the 

 non-development of roots, the whole dentition of this animal 

 presents a most curious similarity to that of the rodents. Some 



