BY C. W. DE VIS. 259 



more than the cutting edge, is very much greater than, in fact 

 nearly twice as great as, that of fig. 5. We are not at liberty to 

 attribute so great a difference to the accidents of function in two 

 individuals or to specific differentiation until we have failed to 

 explain it by reference to any known rule applicable to the case, 

 and such a rule we find maintained in the conditions of the 

 working surfaces by all marsupial herbivores having procumbent 

 incisors in the lower jaw; in these the long slope of the lower tooth 

 working with reciprocating action across the comparatively vertical 

 edge of the upper suffers abrasion to the extent of its motion. 

 Seen in this light, the significance of the relative lengths of the 

 surfaces of abrasion in the teeth figured becomes great, and it 

 points to no other interpretation so likely to be correct as this, 

 that they are due to the interaction of opposed teeth. The 

 inferiority in the width of the tooth, which from this point of 

 view is the upper, affords no ground of objection since this is 

 simply a sign of immaturity ; the tooth is, as Owen rightly inferred 

 from its shape, in course of growth. On the supposition then that 

 this identification of the lower incisor is admissible, the long mid- 

 rib on its concave side, in which its describer is disposed to see an 

 indication of specific difference, becomes merely an item, but an 

 important one, in its diagnosis. 



So far we have deduced all we can, and all that is really neces- 

 sary, from the only figures of the teeth extant, and very probably 

 it is not enough to make good our contention. But evidence in 

 reserve shows, first, that the subject of fig. 5 is really an upper 

 tooth ; secondly, that the characters presented by fig. 7, length of 

 working surface, the longitudinal rib, even a peculiarity in the 

 depression marked b are constant, and, therefore, as to this tooth, 

 we shall have to choose between the probability of its being the 

 lower tooth of S. ramsayi, and the improbability that it is from a 



