592 UNGULATA. 



Sub-ordoi 2. PERISSODACTYLA.* 



Unguligrade forms in which the axis of the hmbs passes through 

 the middle of digit No. 3, which is symmetrical in itself, and the 

 toes of the hind foot are odd in number, never being more 

 than three. The premolars and molars are alike, and the last 

 lower molar is not larger than the others. The first milk 

 molar is generally replaced. There is an alisphenoid canal 

 and the nasals are expanded posteriorly. The number of 

 dorso-lumbar vertebrae is always more than twenty-two. 

 The femur has a third trochanter, the fibula does not articu- 

 late with the calcaneum, and the facets upon the distal face 

 of the astragalus for the navicular and cuboid are very un- 

 equal and its distal surface is flattened. 



These are the differential characters of the living members of 

 the sub-order. They may need some slight qualification in 

 dealing with the extinct members. 



The dentition is frequently complete, i 3 c I p i m f, 

 especially in the Eocene forms ; with increased specialisation 

 there is a tendency to the reduction of the incisors, and a loss 

 of the upper canine and first premolar ; the posterior premolars 

 very commonly resemble the molars (except in some older forms). 

 The grinding teeth are brachyodont in the primitive forms, and 

 generally lophodont ; but occasionally they tend to be bunodont. 

 In the higher forms they are hypsodont, the tubercles being very 

 long, and folded, and the valleys filled in with cement. The last 

 lower molar is commonly without a third lobe. 



The tympanic is small. The orbits are generally open 

 behind (closed only in the Equidae). The dorso-lumbar 

 vertebrae are never fewer than 22 (except in Titano- 

 therium). The scapula is generally without an acromion. 

 The ulna and fibula are complete and free in the less 

 specialised forms, but they become slender distally and 

 incomplete in the higher. The carpus consists of eight bones, 

 the bones of the two rows alternate ; the pollex is always absent. 

 In the tarsus fusion of the cuneiforms may occur, otherwise the 

 bones remain separate ; there is never a trace of digits 1 and 5. 



The stomach is simple, the caecum large, and the gall bladder 



•* G. Cuvier, Eecherches, sur les oss. foss., 3rd ed., Paris, 1846. Cope, 

 The Perissodactyla, Amer. Nat., 1887, pp. 985, 1060. Osborn, The evo- 

 lution of the Ungulf^te foot, Trans. Amer Phil. Soc, n. s , 16, 1889, p. 531. 



