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 Typotherium, 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 



