56 



for example, gained a grinding tooth, for each toe they lost, 

 one in each epoch. In the single-toed existing horses, all the 

 premolars are like the molars, and the process is at an end. 

 Other dental transformations are of equal interest, but this 

 illustration must suffice. 



The changes in the limbs and feet of mammals during the 

 same period were quite as marked. The foot of the primitive 

 mammal was doubtless plantigrade, and certainly five-toed. 

 Many of the early Tertiary forms show this feature, which is 

 still seen in some existing forms. This generalized foot became 

 modified by a gradual loss of the outer toes, and increase in 

 size of the central ones ; the reduction proceeding according to 

 systematic methods, differing in each group. Corresponding 

 changes took place in the limb bones. One result was a great 

 increase in speed, as the power was applied so as to act 

 only in the plane of motion. The best effect of this speciali- 

 zation is seen to-day in the Horse and Antelope, each repre- 

 senting a distinct group of Ungulates, with five-toed ancestors. 



If the history of American Mammals as I have briefly 

 sketched it, seems as a whole incomplete, and unsatisfactory, 

 we must remember that the genealogical tree of this class has its 

 trunk and larger limbs concealed beneath the debris of Meso- 

 zoic time, while its roots doubtless strike so deeply into the 

 Paleozoic that for the present they are lost. A decade or two 

 hence, we shall probably know something of the mammalian 

 fauna of the Cretaceous, and the earlier lineage of our existing 

 mammals can then be traced with more certainty. 



The results T have presented to you are mainly derived 

 from personal observation ; and since a large part of the higher 

 vertebrate remains found in this country have passed through 

 my hands, I am willing to assume full responsibility for my 

 presentation of the subject. 



For our present knowledge of the extinct Mammals, Birds 

 and Reptiles of North America, science is especially indebted 

 to Leidy, whose careful, conscientious work has laid a secure 



