BY C. W. DE VIS. 261 



small tooth shown in PL xxii. fig. 2, a tooth in a much less advanced 

 stage of growth than that of Owen's fig. 5, and so distinctly diffe- 

 rent from the adult tooth now figured as to excite a doubt in his 

 own mind as to their specific co-identity. The doubt vanished as 

 soon as the relative extents of their working surfaces suggested 

 community of causation between them and similar effects else- 

 where. The length of this surface in the young tooth is 31 mm., 

 more than twice its length in the adult upper tooth, but consider- 

 ably less than in the adult lower tooth represented by Owen, fig. 7. 

 Bat the breadth of the tooth itself at the fere end is but 16 mm. 

 against 35 mm. in the type adult, so that its working surface is 

 even longer in proportion to its width than in the adult state. 

 The identity of this tooth with Owen's subject, fig. 7, is established 

 by the presence of the longitudinal rib, and the constancy of this 

 character again is deduced from a third appearance of it in Owen's 

 fig. 2. Tn the depression b of fig. 7, and of PI. xxii. fig. 2, infra, 

 we have a proof of equal persistency. One side of the abraded 

 surface (the left) sends backwards, as is shown in the figures, a 

 tapering tongue which ends in a point depressed in the outer edge 

 of this face of the tooth, and more than half of the surface of wear 

 on the inner side loses its smooth flat character posteriorly and 

 becomes a coarsely roughened depression which in the young tooth 

 is seen to be caused by the direct chopping impact of the sharp 

 edge of the upper tooth on its surface, in which it has cut distinct 

 notches. A similar depression, accompanying a lateral tongue of 

 abraded surface on the opposite side, occurs in the immature upper 

 tooth as figured by Owen, but the depression there is small, smooth 

 and limited to the edge ; the surface, moreover, has no trace of 

 incisive action behind the regular surface of wear. There are thus 

 three features constantly present in these teeth which are absent 

 from the others, and one of these features, an extended area of 

 abrasion, is normal to the lower incisors of other marsupials. The 

 differently conditioned teeth prove to be upper teeth. The plain 



