12 VEKTEBRATA. 



The lacustrine deposits of the Rocky Mountain region have fur- 

 nished great variety and numbers of Tertiary ungulates ; which 

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

 that all the existing genera of Perissodactyla, and the Deer, Cartel 

 and Hog families among Artiodactyla are true American types, and 

 might have populated the Old World by migration. 



SECTION A-PERISSODACTYLA. 



These are the oldest and most abundant ungulates of the Eocene, 

 and as a group are less specialized than the Artiodactyla, although 

 the horse, in feet and teeth, is the most highly differentiated species. 

 In all the living species the hind feet have an odd number (3 or 1) 

 of toes, and the Tapir is the only one with an even number (4) on 

 the fore feet. The axis of the limbs passes through the third digit. 

 In living species there are never less than twenty-two dorso-lumbar 

 vertebrae, and the femur has a third trochanter, and the horns, if 

 present, are placed on the median line of the head, and are not sup- 

 ported on horn cores. In the extinct Menodus [Titanotherhm}) 

 there are a pair of horn cores placed transversely. The stomach is 

 simple and the caecum large. 



There are but three families now living, represented by the Horse, 

 Rhinoceros and Tapir, but these are closely connected by numerous 

 extinct forms. Cope enumerated (1883) one hundred and ninety- 

 two species, of which nineteen are living. He regards his System- 

 odon and Ectocion as parent types. 



FAMILY EaUID^. 



The complete pedigree of the Horse has been traced by Marsh, 

 from the little Eohippus, of the lower Eocene, with four toes, and 

 the rudiment of the first on the fore feet, and three on the hind 

 feet, through seven intermediate genera of successive horizons in 

 the Tertiary to the modern genus, Equus, in the upper Pliocene. 

 Miohippus, from the upper Miocene, resembles Anehitherium 

 Leidy, and Protohippus of the lower Pliocene most resembles 

 Hipyparion. Marsh finds some forty extinct species in this group. 

 It is certain that the Horse originated in America, and roamed over 

 both continents in Post-Tertiary time, and then for an unknown 

 reason became extinct. 



