lS93-] FAIRCHILD — EVOLUTION OF UNGULATE MAMMALS. 207 



The division of this order, proposed by Owen, into Artiodactyla 

 (even-toed), and Perissodactyla (odd-toed), applies to the extinct 

 forms, which as early as the lowest Eocene were thus separated, 

 notwithstanding their generalization in other respects. The present 

 tendency of the stvidy is to regard the Artiodactyla and the Perisso- 

 dactyla, not as constituting one order, but as two parallel series 

 derived from the Condylarthra of Cope, the primitive type of hoofed 

 mammals. P/ienacodus, found at the base of the Eocene in America, 

 having a small brain, five toes and tuberculated molars, possesses 

 characteristics which point to the primitive ungulate type. 



In the evolution of the present Ungulates there are two elements 

 of special interest. The primitive ancestors of the order were 

 probably omnivorous, like the existing pig, with tuberculated (buno- 

 dont) molars. But the specialized forms, as the horse of the odd- 

 toed, and the ruminants among the even-toed, have developed molars 

 better fitted for grinding, which have the enamel disposed on the 

 effective surface in double crescents (selenodont dentition), whose 

 convexity is turned inwards in the upper teeth and outwards in the 

 lower. The other factor in the evolution was the relation of the 

 small bones of the wrists and ankles to the surviving digits. In the 

 loss of the side digits, and the enlargement of the central ones, it 

 became necessary for the latter to either aopropriate the carpal or 

 tarsal bones belonging to the side digits, or for their own small bones 

 to become properly enlarged. Nature employed both methods ; but 

 it has been shown by Kowalevsky, the Russian naturalist, that the 

 first or appropriative method was the better, and that all the species 

 in both sections in which the latter (inadaptive) plan occurred have 

 become extinct. 



The lacustrine deposits of the Rocky Mountain region have 

 furnished great variety and numbers of Tertiary ungulates, which 

 have been studied by Leidy, Cope, Marsh and Scott. They have 

 found that all the existing genera of Perissodactyla, and the Camel 

 and Peccary among Artiodactyla are true American types, and 

 might have populated the Old World by migration. 



Artiodactyla, In this division the digits are four or two in 

 number, and the axis of the limb passes between the third and fourth, 

 which make a symmetrical pair, and by their compressed form have 

 suggested the term "cloven-footed." The femur has no third 

 trochanter. The dorso-lumbar vertebrae are usually nineteen. The 

 true horns are in transverse pairs, with osseous horn-cores. The 

 antlers of the Cervidae are themselves osseous, and deciduous, and 



