464 LAND MAMMALS IN THE WESTERN HEMISPHERE 



long, toothless gap behind the incisors. The premolars were 

 smaller and simpler than the molars, and the anterior ones were 

 very small and were frequently shed at an early stage, making 

 the number of these teeth variable in different specimens. The 

 upper molars also were of quite simple pattern ; the broad and 

 smooth external wall showed no distinct signs of a division 

 into cusps, and from it arose two oblique transverse ridges ; the 

 deep cleft or valley which separated these ridges was divided 

 and made Y-shaped on the grinding surface by a prominent 

 spur from the outer wall between the two principal crests. 

 The lower molars were composed of two crescents, one behind 

 the other, of which the posterior one was very much longer, 

 and both were very narrow transversely. 



The skull had shortened nasal bones, an indication that 

 some sort of a proboscis or prehensile upper lip was present. 

 There was no trace of a horn, and the general aspect of the 

 skull was not unlike that of one of the hornless rhinoceroses, 

 except for its great vertical depth ; the sagittal crest was very 

 short and had almost disappeared. The auditory apparatus 

 was very extraordinary, though it can hardly be described 

 without an undue employment of anatomical terms ; suffice it 

 to say that in addition to the usual outer ear-chamber, formed 

 by the inflated tympanic bone, there was a second chamber in 

 the rear wall of the skull, communicating with the first by a 

 canal. This arrangement would seem to imply an unusual 

 keenness in the sense of hearing. The external entrance to the 

 ear was placed very high up on the side of the head, as in the 

 pigs and in many aquatic mammals, suggesting that \Toxodon 

 was more or less amphibious. The anterior, or symphyseal, 

 region of the lower jaw was very broad, flattened and shovel-like, 

 hardly projecting at all below the plane of the lower incisors. 



The neck was short and stout, the body long and extremely 

 bulky, having an immense, almost hippopotamus-like girth ; 

 the spines of the anterior dorsal vertebrae were very long, mak- 

 ing a high hump at the shoulders. The limbs were short and 



