8 THE HORSE FAMILY. 



comparatively late date to which that species survived, there is no 

 vahd reason why Hijypidium and Onohippidium should not have 

 survived till Cabot^s time, especially as their hoof's have been 

 found in comparativel}/ fresh condition alongside the remains of 

 the Ground -Sloth. 



While the skull of Hippidium shows no marked depression in 

 front of the eye-socket, that of Onohippidium (fig. 4) has an 

 enormous pit in this position, with a smaller and partially 

 detached one in front. 



_ -, - In the same case with the skull of Onohippidium 



_ ,. . „ are exhibited a few specimens (N.H. 34) illus- 



Extinct Horses. . . .i . . \ A-rc ^u 



tratmg the structural diiierences m the upper 

 cheek-teeth of some of the later members of the Equidce, and also 

 the marked difference between an unworn and a worn molar of 

 Equus cahallus. The specimen marked A (fig. 6, A) is an unworn 

 molar tooth of the latter species, with the infoldings of the 

 crown not yet filled with the cement, which is developed later. 

 D (fig. 6, D) shows the condition of a similar tooth which has 

 been some time in use. The summits of the columns coloured 

 red in A have been worn away in D so as to expose the dentine 

 or ivory (red) forming the interior of the tooth ; the infoldings 

 on the crown, of which the central ones are converted into islets, 

 being filled with cement (blue) . The enamel, forming the proper 

 external surface of the tooth, is left of the natural colour. 

 Specimen C (fig. 6, C) is a half-worn tooth of the above-mentioned 

 extinct South American Hippidium, in which the two disks 

 (anterior and posterior pillars) on the lower border coloured red 

 are more alike than in Equus ; the whole crown of the tooth being 

 also shorter. Specimens B and E are respectively slightly worn and 

 half-worn teeth (fig. 6, B & E) of the European Three-toed Hipparion. 

 In these the anterior pillar (a) is isolated from the rest of the 

 crown, thus indicating that the genus is off the direct line of 

 ancestry of the modern representatives of the Horse family. 

 n II ■!. Although it is unnecessary to discuss the «:eneral 



Callosities or * , ... ° . 



, I " structure of the EquidcBj it is important to mention 



that all members of the Horse tribe have a 

 bare patch of hardened skin on the inner side of the fore-leg, 

 situated some distance above the carpus, or "knee." In the 

 Horse a similar but smaller callosity, or " chestnut " generally. 



