THE TOXODONTS 



"53 



certain features now common to the Even-Toed group, which have probably been 

 inherited from common ancestors. 



While, as aforesaid, the more generalized members of the suborder ap- 

 proximate in the structure of their teeth and feet to the Odd-Toed group, the 

 specialized forms assume a more or less Rodent-like type of dentition and limb 

 structure, which must probably be regarded as an instance of parallelism. It 

 may be added that, from the retention of clavicles, these Rodent-like types must 

 be derived from some form less specialized than the toxodon, in which those bones 

 have disappeared. 



The toxodon was of the size of a large rhinoceros, and characterized by the 

 long and curved crowns of its molar teeth, which continued to grow throughout life. 

 There were only two pairs of incisor 

 teeth and no. tusks in the upper jaw, 

 although in the lower jaw the full 

 number of these teeth were developed. 

 The feet were furnished with three toes. 



This genus occurs in the superfi- 

 cial deposits of Argentine, but is re- 

 placed in the Miocene Tertiaries of 

 Patagonia by certain allied forms 

 known as nesodons, which may be 

 briefly defined as including toxodonts 

 of medium or small size, in which the 

 limbs, and probably also the neck, 

 were relatively longer and more slen- 

 der than in the typical genus; while 

 all the teeth, with the exception of 

 the second upper and third lower in- 

 cisors, developed roots in the adult 

 state, and the upper molars were of 

 a type approaching that of the Odd- 

 Toed group, with a distinct posterior 

 valley, and the middle column form- 

 ing a distinct lobe projecting into the 

 median valley. The second upper 

 and the third lower incisors formed 

 a pair of permanently-growing tusks, 

 which were, however, not fully de- 

 veloped till late in life. 



Even more strange than the toxodon was the smaller typothere of the same 

 region, which represents a remarkable approximation in the characteristics of its 

 skull and teeth to the Rodents. While the molars were not unlike those of the 

 toxodon, the upper incisors were reduced to a single chisel-shaped pair, and there 

 were no tusks in either jaw. The lower jaw carried one large pair of chisel-like 

 incisor teeth, behind which there came a much smaller second pair. The typothere 

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UNDER SURFACE OF SKULI. OF THE NESODON. 

 (One-fourth natural size.) 



