293 



the teeth of all the mammals of the Lance Creek stage, except those of 

 the Allotheria. are triangular, showing that the possessors were either in- 

 sectivorous or flesh-eating in their habits. 



On the other hand, there are several genera of Puerco mammals that 

 possess a well developed hypocone and internal cingulum. In some cases, 

 where the hypocone had no great development, the hinder internal part of 

 the tooth had swollen so as to reduce much the gap between the successive 

 teeth and produce a broad triturating surface. In Polymastodon, which 

 must have been a vegetarian, an extensive triturating surface was secured 

 in another way. It presents a great advance over the teeth of any of the 

 Lance Creek Allotheria. If it is considered how slowly changes in tooth 

 structure had advanced during the Mesozoic era we must conclude either 

 that a considerable interval had elapsed between the Lance Creek epoch 

 and that of the Puerco or that the animals of the latter were not de- 

 scendants of the former. 



There are important differences between the mammals of the Lance 

 Creek beds and those of the Puerco as regards the size attained. Most of 

 the former are of insignificant proportions, resembling in this respect those 

 of the Jurassic ; while many of those of the Puerco are large. Further- 

 more, there was in the mammals of the Puerco a far greater variety of 

 form, structure, and systematic relationships than among those of the 

 Lance Creek mammals. Of the latter, there have been described about 

 twenty-five genera and about forty-five species, most of them by Marsh. 

 Osborn has regarded himself as justified in reducing these to about ten 

 genera, these representing a very few families. From the Puerco Matthew 

 (Bull. 361, U. S. Geol. Surv., 1909, p. 91) recognizes twenty-nine species, 

 belonging to eighteen genera and nine families. To what extent this in- 

 creased diversification of the mammalian life of the Puerco is due to im- 

 migration we can not now tell ; but it does not seem to be necessary to 

 assume that it was due to invasion of mammals from some other region. 

 For, in view of the interval between the two formations that is indicated 

 by the plants and reptiles, it is possible that the Puerco mammals are the 

 direct descendants of those of the Lance Creek epoch. 



In case there was no serious interruption in deposition between the 

 Lance Creek beds and the Puerco and Fort Union, one might expect to 

 find close relationships between the reptiles of the two levels. Crocodiles 

 are not abundant in either and, so far as known, no species passes from 



