UNGULATA. 



319 



the province of Buenos Aires, Argentina. M. boliviensis is a small allied 

 species known by parts of the skeleton from the highlands of Bolivia. 



Sub-Order 9. Perissodactyla. 



The existing odd-toed hoofed animals, or PERISSODACTYLA, 

 are represented by the three distinct families of tapirs (Tapi- 

 ridse), rhinoceroses (Rhinocerotidse), and horses (Equidse), which 

 are proved by the known fossils to have become gradually 

 differentiated during the Tertiary period. So large a number 

 of their extinct predecessors have indeed been discovered, that 

 it is already impossible to make a satisfactory classification of 

 the sub-order as a whole. They are all digitigrade quadrupeds, 



FIG. 181. 



Astragalus of a Perissodactyl Ungulate (A) and an Artiodactyl Ungulate (B), 

 anterior aspect. cb. t facette for cuboid; n, facette for navicular; t, facette 

 for tibia. 



with the axis of both feet passing through the digit no. ill 

 (hence mesaxonic)', and the bones of the two series in the 

 carpus alternate or interlock, while the astragalus (fig. 181 A) 

 in the tarsus is always flattened below, articulating not only 

 with the navicular but also partly with the cuboid. There is 

 no clavicle. The femur always bears a third trochanter. In 

 other features of the skeleton there are many variations; but 

 the general tendency of evolution is quite clearly that already 

 described as characteristic of the Ungulata in general (p. 287), 

 and it is well illustrated in the case of the horses in the 



