FOSSIL MAMMALIA. °° 



and lower grinders, but that they deviate from one another in a very similar manner 

 to that above described in the Toxodon. In this comparison attention should be 

 confined to the course of the external enveloping layer of enamel, leaving out of 

 consideration the central crescentic islands of enamel which constitute the addi- 

 tional complexity of the Horse's grinder. Viewing then the course of the external 

 coat of enamel on the worn surface of the tooth, we find it describing on the outer 

 side of the tooth in the upper jaw an undulating line,— a middle convexity being 

 situated between two concavities; on the inner side of the tooth one fold of enamel 

 penetrates to the middle of the tooth, and on each side of this there is a smaller 

 fold. But in the lower jaw the line of enamel on the outer side of the tooth, 

 instead of merely bending outwards midway in its course, is reflected a little 

 way inwards ; while on the opposite, or inner side of the tooth, the enamel sends 

 two extensive folds into the substance of the tooth, opposite to the interspace of 

 which the shorter fold projects from the outer side. Now, on the supposition that 

 the fragment of the lower jaw here described belongs to the Toxodon, the kind and de- 

 gree of difference in the complexity of the grinding surface of the teeth in the upper 

 and lower jaw, are remarkably analogous to those which exist in the Horse. I have 

 only further to remark that in the Horse the inflected folds of enamel, instead of 

 being simple and straight with the two constitutive layers in apposition, as in the 

 Toxodon, are irregular in their course, with coementum intervening between the 

 constitutive layers, which also diverge from each other at their angle of reflection, 

 so as to augment the amount of dense material which enters into the composition 

 of the tooth. 



Many analogous examples will readily occur to the experienced comparative 

 anatomist. The Horse has been adduced as one to which reference can very readily 

 be made ; but I would also cite the Sumatran Rhinoceros, the skull of which, in 

 the Hunterian collection, has already been alluded to. In this species the ante- 

 rior grinders, in both jaws, are small and simple, and increase in complexity as 

 they recede backwards. The third superior grinder (fig. 8, PI. V.) presents a 

 single fold of enamel, reflected obliquely forwards from the inner side half-way 

 across the tooth ; the outer line of enamel describes a simply undulating line. The 

 opposite grinder of the lower jaw (fig. 9, PL V.) has only one-half the breadth of 

 the upper one, but has its grinding surface further complicated by having two in- 

 flected folds of enamel from the inner side, and one shorter and broader fold from 

 the outer side. This tooth, therefore, presents a close resemblance to one of the 

 posterior grinders of the lower jaw of the Toxodon, but differs essentially in being 

 of limited growth, and consequently in having fangs.* 



* Besides the relation to food requiring much comminution, which teeth with persistent pulps bear, they 

 are also connected with the longevity of the individual. The term of life in a herbivorous animal, with grinders 



F 



