620 HISTORICAL GEOLOGY 



accustomed, is seen in a hoofed mammal, a member of the oreodont family (Agrio- 

 chaerus). In this Oligocene creature the feet and limbs are modified in such a way 

 as apparently to enable it to climb trees as easily as a jaguar or other large cat. The 

 hoofs are so narrow as to be actually converted into a sort of claw, and the wrist and 

 ankle joints are modified in such a way as to make the wrists and ankles as flexible as 

 those of a cat. 1 



Insectivores. — This primitive group occupied an important place 

 in the Eocene, since which time it has dwindled in numbers and is 

 now represented by a few survivors inhabiting, for the most part, 

 uncongenial regions, or else protected by spiny armor, or of sub- 

 terranean habits. It is represented by the mole, hedgehog (not 

 the porcupine), shrew, and other small animals that feed largely 

 on insects and worms. Insectivores have remained simple in their 

 organization since their introduction and are the least altered of the 

 great branches. Among their generalized characters are the smooth 

 brain, five-clawed toes, the habit of walking with the whole or greater 

 part of the soles to the ground (plantigrade), and other less con- 

 spicuous features. The group is so generalized, in fact, that it is 

 difficult to characterize it without a too technical description. Per- 

 haps the most striking feature of some living genera is the elongated, 

 or proboscis-like, nose. It is possible that this group more nearly 

 represents the characters and habits of the primitive true mammals 

 (Eutheria) than any other now living. There seems to be little doubt 

 that bats are descendants of primitive members of this group. 



Rodents (Gnawing Animals). — Rodents are first known from the 

 Eocene, before the close of which epoch they had acquired practically 

 all their present characteristics. With the exception of their powerful 

 gnawing teeth (incisors), rodents, in the past and present, have been 

 animals of simple structure. Their brains are smooth, and the race 

 has apparently changed in no essential feature since Eocene times, 

 with the exception of their teeth which have been slightly reduced 

 in number, and the grinders (molars and premolars), in some species, 

 have become perhaps as highly developed as those of any other class. 

 Before the close of the Oligocene, squirrels, marmots, beavers, rabbits, 

 pocket gophers, and others were present. 



A burrowing rodent with horns, which appears in the Miocene and early Pliocene, is 

 interesting as showing the possibility of variation. It seems to have been much better 

 adapted for Jigging than existing gophers, but of what use the horns could have been 



'Matthew, W. D., — A Tree-Climbing Ruminant: Am. Museum Jour., Vol. n, ion, 

 pp. 162-163. 



