60 THE DESEADO FORMATION OF PATAGONIA 
which, as they have advanced, have developed a bifurcated 
fold on the inner side of the upper molars, which in its 
complete development makes the upper molars three-lobed, 
as is seen in the typical 7ypotherium, representing the 
end of the series up in the Pampean formation. These 
relationships may be expressed graphically as in fig. 26. 
ADAPTATIONS 
Most striking of all the typothere peculiarities, is the 
development of the first upper and lower incisor into per- 
manently growing teeth, having the enamel reduced to the 
anterior side only, making thus a self-sharpening tooth 
similar to that of rodents. Such teeth are characteristic 
of gnawing forms and would indicate that the form lived, 
in at least a considerable part, on bark and twigs. In the 
eating of such food and breaking up the wood cells for the 
contained protoplasm and _ starch, an immense amount 
of chewing is involved, followed by a rapid wear of the 
molars. This is met, as is characteristic in rodents and 
grass eaters, by the development of first high-crowned, then 
permanently growing molars. In acquiring the perma- 
nently growing tooth, some of the irregularities of the crown 
are lost, others which are deep-seated enough to affect 
the tooth even to the root are maintained, so that especially 
the external and internal infoldings become a _ persistent 
part of the tooth, having been impressed into the dental 
papilla. A further supplement to the resistant character 
of the teeth is seen in the development, in the most ad- 
vanced types, of a cement layer on the outside of the molars, 
a feature apparently also a part of permanently growing 
roots. 
The feet are generally those of a running type, but a 
single phylum has acquired the hopping habit. 
The above features seem to indicate a more special 
adaptation than grass feeding. From the aspect of the 
