FOSSIL MAMMALIA. 



31 



interest to find that the presence of denies scalprarii at the anterior part of the 

 mouth has not been necessarily limited to Mammalia of small size. 



The position of the pulps of these incisors, in close proximity with the ante- 

 rior grinders, corresponds with the position of the pulps of the incisors in the 

 upper jaw of the Toxodon, and indicates, in conjunction with the size of the pulps, 

 that a considerable extent of the inferior incisors was lodged in the substance of 

 the anterior part of the jaw. It is most likely that no vertically directed tooth 

 would be developed in the part of the jaw so occupied by the curved bases of 

 the incisors, and hence a diastema or toothless space would intervene between 

 the molars and incisors of this lower jaw, as in the upper jaw of the 

 Toxodon. 



It is interesting, also, to observe, that as the deviations from the Rodent type, 

 which occur in the cranium of the Toxodon, are the same, in some instances, 

 as those which obtain in the Wombat ; so we find a corresponding deviation in the 

 size and relative position of the inferior incisors, which, as in the Wombat, termi- 

 nate anterior to the molar teeth, instead of extending backwards beyond the last 

 grinder, as in most of the true Rodents. The Capybara presents the nearest 

 approach to this structure, the pulps of the inferior incisors being situated opposite 

 the interspace of the first and second grinders. 



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

 Toxodon, had persistent pulps, as is proved by the conical cavity at their base, as re- 

 presented in fig. 3 ; they consequently required a deep socket, and a corresponding 

 depth of jaw to form the socket and protect the pulps. In order to economise 

 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, 

 we find the molars and their sockets are curved, but in a less degree than those of 

 the upper jaw of the Toxodon. They correspond, however, with the superior 

 molars of the Toxodon in the antero-posterior diameter, in being small and simple 

 at the anterior part of the jaw, and by increasing in magnitude and complexity as 

 they are situated more posteriorly. They are, however, narrower from side to 

 side ; but supposing them to belong to the Toxodon, it would agree 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. 



The first grinder, in the lower jaw here described (PI. V. fig. 2), is of small 

 size and simple structure, being surrounded with a coating of enamel of uniform 

 thickness, and without any fold penetrating the substance of the tooth. It is more 

 curved than any of the other molars, and appears to have differed from the external 

 incisor only in its entire coating of enamel and direction of growth; it is interesting, 

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



