THE TOXODONTS 22 
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 togrind. 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, | 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 
