SUBORDER c PERISSODACTYLA 127 



the third digit is strongly developed; keels extending over the entire distal surface 

 of the metapodials. 



Epitherium Ameghino (Fig. 162). Second and fourth toe short and thin. 

 Earliest Pleistocene. Monte Hermoso, Argentina. jEoauchenia Ameghino. 

 Same locality. Crowns of the teeth high. 



Diadiaphorus (Fig. 161) and Froterotherium Ameghino. Miocene and 

 Pliocene. Tridactyl. Isolated pillars on the inner side of the semicircular 

 ridge of the lower molars. Ulna reduced. 



Brachytherium Ameghino. Pliocene of Argentina. 



Licaphrkmi Ameghino, Tetramerorhinibs Ameghino. Miocene of Patagonia. 

 Thoathermm Ameghino (Fig. 163). Same locality. With only one toe. 



Eoproterotherium, Deuterotherium Ameghino. Lower Miocene. Pyrotherium 

 beds of Patagonia. 



Family 4. Adiantidae Ameghino. 



Adiantus Amegh. (Fig. 164). Proadiantns Ameghino. Miocene of 

 Patagonia. Only lower jaw is known, which has very close tooth rows, 

 3.1.4.3.; incisors and canines are chisel-shaped, molars rather high and 

 composed of two crescents and inner cusps. Systematic position uncertain. 



Suborder C. PERISSODACTYLA Owen. (Mesaxonia Marsh.) 

 Odd-toed Ungulates.^ 



Digitigrade hoofed quadrupeds with the middle digit much more developed than 

 the others ; usually with three digits in the pes, from three to four in the manus, 

 sometimes with only one toe in each foot. Astragalus with deeply grooved pulley-like 

 articulation, flattened distally. Carpal and tarsal hones alternating. Dentition 

 usually complete. Cheek teeth lophodont, more rarely hunodont. Femur with third 

 trochanter. Fibula not articulating with calcaneum. 



The perissodactyls constitute a division of herbivorous ungulates extremely 

 abundant in forms, only three genera (sensu lato) of which still exist — Tapirus, 

 Rhinoceros and Equus. In former geological periods, however, that is, from 

 the earliest Eocene to the Pleistocene, they had a wide distribution and were 

 represented by numerous genera and species. They are usually characterised 

 by the great development of the third or middle digit in the fore- and hind- 

 feet, which in the most specialised forms [Equus) alone bears the weight of 

 the body, and in almost all cases the main axis of the extremities passes 

 through it. 



The skull acquired an elongated form through the excess in weight of 

 the facial bones over the brain case. The occiput slopes abruptly, the con- 

 dyles are convex transversely, and the large temporal fossae are surmounted 

 by a temporal crest. The nasal bones stand out freely above the narial 

 apertures, which open laterally, extend far backward, and are bounded below 



^ Cope, E. D., The Perissoflactyla. Amer. Nat., vol. xxi., p. 985, 1887. — Osboni, H. F., 

 Mammalia of the Uiuta P"'ormation. III. The Perissodaetyla. Trans. Amer. Phil. See, vol. xvi., 

 part iv., 1889. — The Evolution of the Ungulate Foot. Ibid. — Oshorn, H. F., and Wortman, J. L., 

 Perissodactyls of the Lower Miocene White River Beds. Bull. Amer. Mns. Nat. Hist., pp. 343- 

 375, 1895. — (hveii, Richard. Quart. Journ. Geol. Soc. London, vol. iv., p. 103, 1847. — StehUn, 

 H. G., Die Saugetiere der Schweiz. Eocan. Abh. Schweiz. Palaont. Gesellsch., 1903, 1904, 1906. 



