6 9 6 THE POPULAR SCIENCE MONTHLY. 



Another of the interesting changes in mammals during Tertiary 

 time was in the teeth, which were gradually modified with other parts 

 of the structure. The primitive form of tooth was clearly a cone, and 

 all others are derived from this. All classes of Vertebrates below 

 mammals, namely, fishes, amphibians, reptiles, and birds, have coni- 

 cal teeth, if any, or some simple modification of this form. The Eden- 

 tates and Cetaceans with teeth retain this type, except the zeuglodonts, 

 which approach the dentition of aquatic Carnivores. In the higher 

 mammals, the incisors and canines retain the conical shape, and the 

 premolars have only in part been transformed. The latter gradually 

 change to the more complicated molar pattern, and hence are not 

 reduced molars, but transition forms from the cone to more complex 

 types. Most of the early Tertiary mammals had forty-four teeth, and 

 in the oldest forms the premolars were all unlike the molars, while 

 the crowns were short, covered with enamel, and without cement. 

 Each stage of progress in the differentiation of the animal was, as a 

 rule, marked by a change in the teeth ; one of the most common being 

 the transfer, in form at least, of a premolar to the molar series, and a 

 gradual lengthening of the crown. Hence, it is often easy to decide 

 from a fragment of a jaw to what horizon of the Tertiary it belongs. 

 The fossil horses of this period, 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 illus- 

 tration 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 specialization is seen 

 to-day in the horse and antelope, each representing 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 Mesozoic time, while its roots doubt- 

 less 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. 



