SUBORDER C 



PERISSODACTYLA 



159 



forms, incisors small, weak ; in older species, very well developed. Diastema between 

 canines and premolars very short: In older forms, all premolars less complex ; in 

 later species, the two posterior premolars more complex. Superior molars with 

 y^ -shaped ectoloph and tioo conical internal cusps; inferior molars consisting of 

 two V-shaped crescents, the inner horns of which at their union usually form a 

 bicuspidate pillar. Third lower molar with large third lobe. Manus tetradactyl : 

 pes tridactyl. Hoofs moderately broad, flattened below. 



This wholly extinct family, which was limited principally to the Eocene 

 and Oligocene of North America, includes large robust ungulates that in 

 external appearance chiefly recall the tapir and rhinoceros, but sometimes 

 nearly equalled the elephant in size. As their dentition indicates, they fed 

 on succulent vegetation. The cheek teeth have very low crowns ; the two 

 external cusps of the superior molars are V-shaped, and through their union 

 \ form a W-shaped ectoloph with a parastyle and mesostyle. The two conical 



Fig. 215. 



Palaeosyops major Leidy. Middle Eocene (Bridger beds), Green River, Wyoming. A, Upper cheek teeth. 



B, Lower jaw. i/;j. 



internal cusps generally remain distinct or are joined to the ectoloph by a 

 slight ridge ; the protocone is always stouter than the hypocone. The superior 

 canines are only moderately robust and either immediately follow the cheek 

 teeth or are separated from them by a very short diastema. The Eocene 

 genera have the full number of incisors, but in Oligocene forms these teeth 

 are deciduous. 



The posterior premolars are distinguished from the molars by their smaller 

 size and less complex construction ; the two anterior premolars are somewhat 

 simpler. With the exception of the first, all the cheek teeth in the mandible 

 are composed of two V-shaped crescents. The skull is elongated and low ; 

 the brain-cavity is small. In later forms, blunt conical bony protuberances 

 arise on the posterior border of the nasal bones ; these probably differ in size 

 in the male and female. The extremities are similar to those of the tapir. 

 The carpus is broad and the ossicles of both series really alternate, although 

 there is very little lateral displacement. The manus has four functional 

 digits ; the pes, three. The calcaneum presents a facette for the fibula. 

 The ungual phalanges are flattened distally and somewhat expanded. 



