ORDER UNGULATA. 



135. 



the Lophodont plan (fig. 1228); that is, there is an outer longitu- 

 dinal wall, from which two transverse ridges proceed at right angles 

 towards the inner border of the crown. In the brachydont forms 

 this structure is perfectly simple, but in those genera with very 

 hypsodont teeth it is so complicated by foldings and involutions 

 that it is not always easy to trace the original plan. The crowns 

 of the lower true molars consist in their simplest structure of two 

 transverse ridges (as in the Tapir), but these ridges may be curved 

 into crescents (as in the Rhinoceros), or complicated by foldings 



Fig. 1225. — Right manus of (a) Tapirus, and (b) 

 Rhinoceros. Reduced. (After Flower.) 



Fig. 1226. — Dorsal or anterior 

 view of the left femur of Rhino- 

 ceros. The median projection on 

 the right side of the figure is the 

 third trochanter. Reduced. 



and convolutions (as in the Horse). The transition from the sim- 

 plest brachydont to the most specialised hypsodont dentition is 

 accompanied by a reduction of the number of the digits from four 

 or three to one ; that one being the third, or middle, of the typical 

 series of five. 



The Perissodactyla have suffered considerably more in proportion 

 to their numbers than the Artiodactyla by the extinction of generic 

 and family types ; the existing genera being at the present day 

 reduced to three, which are the types of as many different families. 

 Some writers have suggested that this extinction of types is owing 

 to the Lophodont plan of molar structure being less readily suscep- 



