330 LAND MAMMALS IN THE WESTERN HEMISPHERE 



moderately high crowns, yet they are purely brachyodont, 

 except in the grazing, broad-lipped African species (0. simus), 

 in which they may fairly be called hypsodont. The external 

 wall of the tooth is broad and nearly smooth, not divided into 

 cusps, as it is in the horses and tapirs, and the two transverse 

 crests, which in the tapirs are directly transverse, are very 

 oblique. In all the existing species additional complications 

 are given by the short spurs, which project inward from the 

 outer wall or from the transverse crests. The lower molars 

 are formed each of two crescents, one behind the other, but 

 their arms or horns are angulate, not curved as they are in 

 other perissodactyls which have crescentic lower teeth. 



The upper surface of the skull is very concave in the antero- 

 posterior direction and very broad over the cranium, where 

 there is no sagittal crest. The nasal bones are immensely 

 thick and strongly arched, with the convexity upward ; both 

 this arching of the nasals and the fore-and-aft concavity of the 

 skull are devices for giving a strong and solid attachment to 

 the great nasal horn, for the attachment of which these bones 

 have an extremely rough surface, and in the two-horned species, 

 a second roughened area on the forehead marks the place 

 of attachment of the frontal horn. The bones of the cranium 

 are very thick, but lightened by the many chambers which 

 traverse them. The articulation of the lower jaw with the 

 skull is in some respects unique among mammals ; the post- 

 glenoid process is a long spike, which fits inside of a bony 

 lump (the postcotyloid process) behind the condyle of the lower 

 jaw, and the posterior margin of the latter is greatly thickened. 

 The neck is short and stout, the trunk very long, broad and 

 deep, the long and strongly arched ribs and the widely ex- 

 panded hip-bones providing space for the great mass of viscera. 

 The bones of the limbs are short and very massive ; the humerus 

 has a very prominent deltoid ridge and the femur an unusually 

 large third trochanter ; the bones of the fore-arm and lower 

 leg are separate, as in the massive ungulates generally. The 



