THE TOXODONTS 23 



of permanently growing incisors. They are typical of 

 gnawing animals, eaters of bark, twigs, and possibly also 

 leaves, the wood and bark being also a hard type of vegeta- 

 tion to grind. In the case of these forms I believe they were 

 feeders on grass and bark. Their feet are developed either 

 for running or hopping and would suggest hard ground 

 for their habitat. 



The Litopterna are typically plains animals, paralleling 

 in their development the horses. The cropping teeth and 

 the grinding molars become progressively longer. The 

 limbs are progressively elongated, the animals walking 

 more and more on the tips of the toes. With this, the 

 metapodials especially and the other limb bones to a less 

 degree, are progressively lengthened. At the same time 

 the side toes are progressively reduced. The teeth indi- 

 cate grass eating; the limbs life on the plains. 



The Rhynchippidae, while not as advanced as the Litop- 

 terna, show cropping front teeth, and the molars develop- 

 ing in depth. The locomotion is semidigitigrade, the feet 

 small, and the number of toes reduced to three. They too 

 must be interpreted as grazing or grazing and browsing 

 animals, living on hard ground. 



The Leontinidae are heavier forms, but with much the 

 same features as Rynchippidae, though less specialized. 

 On account of the broad upper molars and the less special- 

 ization of the dentition, I should feel that these forms were 

 browsers and lived among bushes, but the feet were three 

 toed and semidigitigrade and they seem to have walked 

 on hard ground. 



The Nesodontidae belong to the same type of adaptation 

 as the foregoing family, but have the grinding teeth more 

 complicated, indicative of a more advanced adaptation to 

 hard vegetation. The feet were also adapted to hard 

 ground. 



The Homalodontotheria, the Astrapotheria, and the 

 Pyrotheria were all very large animals, known mostly by 



