TOXODON. 585 



enamel about half a line in thickness, which terminates at 

 the angles between these and the posterior or concave surface. 

 From the relative position of the bases or roots, we may infer 

 that they diverged from each other, like the incisors of the Horse, as 

 they advanced forwards in order to bring their broadest cutting 

 surface into line. That they were opposed to teeth of a corresponding 

 structure in the upper jaw, is proved by their oblique chisel-like 

 cutting edge. 



The molar teeth in this mutilated lower jaw, like those in the 

 upper jaw of the Toxodon platensis, had persistent pulps, as is proved 

 by the conical cavity at their base : they consequently required a 

 deep socket and a corresponding extent of jaw to form the sockets 

 and protect the pulps. In order to economize space and to increase 

 the power of resistance in the tooth, and perhaps also to diminish 

 the effects of direct pressure on the highly vascular and sensible 

 matrix, the molars and their sockets are curved, but in a less degree 

 than those of the upper jaw. They correspond with the superior 

 molars in the long antero-posterior diameter, in being small and 

 simple at the anterior part of the jaw, and by increasing in mag- 

 nitude and complexity as they are situated more posteriorly. They 

 are, however, narrower from side to side, the Toxodon agreeing in 

 this respect with most other large herbivorous Mammalia, the fixed 

 surface for attrition in the upper jaw being from obvious principles 

 more extensive than the opposed moveable surface in the lower jaw. 

 In the first three teeth, which are premolars, the enamel is confined 

 to the outer surface : in the last three or true molars a plate of enamel 

 is also laid upon the middle of the inner surface, and sends one or 

 two simple folds obliquely forwards into the substance of the tooth. 

 (PL 145, fig. 3). 



The first grinder in the lower jaw is of small size and simple 

 structure. It is more curved than any of the other molars, and appears 

 to have diflfered from the external incisor only in its more compressed 

 form and vertical direction of growth : it is interesting, indeed, to 

 find so gradual a transition, in structure, from molar to incisive teeth 

 as this jaw presents ; for the robust incisors may here be regarded as 

 representing molars simplified by the greater deficiency of enamel. 



