Tertiary Vertebrata of the West. 475 



Chalicotheridge have the toes 4-3, and the MacraucheniidaB have the 

 toes 3-3. And secondly come families in which the molars are like 

 the premolars, but with two internal lobes ; and among these the 

 Menodontidas have the digits 4-4, the Palasotheriidaa have the digits 

 3-3, while the Equidaa have the digits 1-1. These families include 46 

 genera and 192 species. The author remarks that the Lophiodontidaa 

 and Chalicotheridse are prevalent in the Eocene ; the Bhinoceridaa 

 and Palasotheriidas characterize the Miocene ; while the Tapiridaa and 

 Equidaa are chiefly found in the latest Tertiary epochs. The follow- 

 ing table exhibits the relations of the families with each other. 



Equidse 



Bhinoceridse Palaeotheriidse 



Hyracodontidse 



Tapiridse 

 Triplopidse ! Menodontidae 



Lophiodontidee ChalicotheriidaB 



1 J 



Hyracotherimse. 



The Lophiodontidse comprises a large number of species, all of 

 which, with one exception, are limited to the Eocene rocks. Among 

 living animals the Tapirs most nearly resemble them. They 

 range in size from that of a Rabbit to the bulk of an Ox. Lophiodon 

 is limited to Europe ; Hyrachyus, Hyracotherium and Pliolophus to 

 Eui-ope and North America ; while Systemodon, Heptodon, Helaletes, 

 and Colonoceras are only found in North America. All these genera 

 are classified and defined by dental characters. Systemodon is allied 

 to Hyracotherium and Pliolophus ; but instead of having the wide 

 diastema behind the canine tooth, seen in Hyracotherium, there is no 

 interspace at all. The dental formula appears to be the same, and 

 the differences are in the form and arrangement of the cusps. The 

 genus is represented by two species. 



Hyracotherium is admirably known from six American species ; 

 it shows three diastemata, behind the third incisor, the canine, 

 and the first premolar in the upper jaw ; and two in the lower 

 jaw, behind the canine, and the first premolar. The species of 

 this genus differ in the relative development of the intermediate 

 tubercles of the upper molars, in the distinctness of the tubercles 

 of the lower molars, the varying lengths of the diastemata, and 

 the development of the cingula. The form of the head is very like 

 Anchitherium, and the cingulum of the upper molar teeth is re- 

 presented in Anchitherium by a ledge, while the principal bones 

 of the limbs are much alike in both genera, so that there are some 

 grounds for suggesting an ancestral relation between them. The 

 coracoid process of the scapula in Hyracotherium is incurved as 

 in Coryphodon and Anchitherium, and is larger than in its allies 

 Triplopus and Hyrachyus. The humerus, especially in its proximal 

 end, has much in common with these genera, and in the simple 

 groove for the biceps is like Tapirus and Anchitherium. The 

 carpus includes eight bones, and is more like that of Hyrachyus than 



