SUBORDER C 



PERISSODACTYLA 



131 



the upper jaw usually retains the full number (four). Only the Eocene 

 genus Lophiodon is remarkable as exhibiting but three premolars above and 

 below. The skull of the tapir is elongated, the snout slender, the brain 

 cavity moderately large. Posteriorly the orbits are not surrounded by bone, 

 the nasal bones stand out freely and in the older forms are very long and 

 massive, but in Tapiriis are short and triangular, leaving space for a short 

 proboscis. The narial opening is large and much prolonged backward ; the 

 postglenoid and paroccipital processes are well developed. The extremities 

 (Fig. 167) have moderate length, are more slender than those of most 

 rhinoceroses, yet are more robust than those of most horses. The radius 

 and ulna remain entirely separate, and are about equally developed. In 

 the carpus, the scaphoid rests on the trapezoid and magnum, the lunar on 

 the magnum and unciform. The magnum bears the principal weight, and is 

 supported by the second and third metacarpals. In all well-known genera, 

 the fore-foot is tetradactyl ; the hind-foot, tridactyl. In the latter, the femur 

 bears a moderately well-developed third trochanter, while the fibula is always 

 complete. The astragalus has a grooved trochlea, distally a large flat 

 articular surface for the navicular and a small one for the cuboid. The 

 structure of the carpus and tarsus exhibits no essential change during 

 Tertiary time. 



Subfamily 1. Lophiodontinae Gill.^ 



Upper and lower molars with two oblique transverse crests. All, or only the two 

 anterior premolars, simpler than the molars. 



Fossil in the Eocene of Europe and North America only. 



Heptodon Cope. Dental formula : ' ' • All premolars simpler than 



the molars. Third lower M with small third lobe. Lower Eocene (Wasatch 

 and Wind Eiver . 



A 



beds); Wyoming. 

 H. ventornm, E. calci- 

 culus Cope. 



Helaletes Marsh 

 (Dilophodon and Des- 

 matotherium Scott). 

 Premolars |-, the last 

 two in the upper jaw 

 with supplementary 

 crest (metalophid). 

 Upper Eocene 

 (Bridger and Uinta 

 beds); North 

 America. 



Chasmotherium 

 Riitimeyer. Pre- 

 molars I", the last two of which are somewhat molariform. Milk cheek 



1 Filhol, 77., Etudes sur les vertebres fossiles d'lssel. Mem. Soc. Geol. France, 1888. — Deperet, Ch., 

 Etudes paleontologiques sur les Loptiiodons du Minervois. Arch. Mus. d'Hist. Nat. Lyon, t. 

 ix., 1903. — Stehlin, H. G., Die Saugetiere des schweizerischen Eocan. Teil I. Chasmotherium, 

 Lophiodon. Abhandl. Schweizer. Palaont. Gesellsch., 1903. — Oshorn, H. i^., and Wortman, J. L., 

 Bull. Amer. Mus. Nat. Hist., N.Y., 1892, 1893. 



Lophiodon isselense Cuvier. Middle Eocene, Issel, Aude. A, Left upper 

 cheek teeth. B, Left lower teeth. 1/2- (After Filhol.) 



