PEOP. W. H. FLO WEE ON THE GENUS MESOPLODON. 427 



height of the tooth at the middle is l"-7, or 43 millims. The base is extended antero- 

 posteriorly to a length of 2"T or 53 millims., and has a rough, spongy, unfinished 

 and laminated edge, without any distinct pulp-cavity. The greatest lateral thickness 

 of the tooth is 6 millims. or 0"'25. 



The old specimen (M. haasti of this communication) has a tooth very closely re- 

 sembling tbat of the adult M. grayi, judging of the latter from Dr. Haast's description 

 and from photographs which he has sent me, and well illustrates the changes which have 

 taken place in passing to maturity. These are : — rounding of the apex, with almost 

 complete destruction of the enamel, except a small patch on the hinder part of the 

 tooth ; great wear of the whole anterior edge or shoulder of the tooth ; and a great 

 increase of the supporting mass or bone, the growth of which is evidently completed, 

 as the lower edge is now firm and rounded, and has developed upon it a number of 

 short root-like processes. The height of the tooth is 3" "3 = 84 millims., its greatest 

 antero-posterior breadth 3"'9 = 99 millims., its greatest lateral width 11 millims. or 

 nearly half an inch 1 . 



This tooth is therefore of altogether a different form from that of the adult 

 M. bidens and europceus, in neither of which does the base enlarge in the antero-pos- 

 terior direction, but forms an oblong figure with parallel sides, the axis of which does 

 not coincide with the axis of the apex. Neither has it any resemblance to that of 

 M. layardi, the highly characteristic form of which has already been pointed out. In 

 If. densirostris, though the exposed portion of the tooth is triangular and has its 

 apex placed nearly vertically, as in M. grayi and M. haasti, the root when complete 

 assumes an oval form, contracting towards the base, and its vertical direction greatly 

 exceeds the antero-posterior. Most unfortunately, in the otherwise nearly perfect 

 adult skeleton of M. australis, these most important and characteristic organs are 

 wanting. They were carried off, as Dr. Hector informs us, by some mischievous boys, 

 by whom the jaw-bone was broken into many pieces in extracting them ; and thus no 

 satisfactory idea even of the form of the alveolus can be obtained. As, however, the 

 whole of the symphysis, with the anterior margin of the left alveolus, is preserved, it 

 can be demonstrated that the situation of the tooth in relation to the mandible was 

 quite the same as in M. grayi and M. haasti, though (in accordance with the general 

 smaller size of the individual) the distance between the apex of the mandible and the 

 tooth is somewhat less than in the latter. 



It is very probable that the fragment of a jaw of a very young Mesoplodon, found on 

 the beach at Kaikoura, and described and figured by Dr. Hector, may belong to this 



1 It has long been suspected that the sexes of Mesoplodon differ greatly in the development of the teeth ; 

 but, owing to the want of materials, this has not been definitely proved. Of M. grayi, however, Dr. Haast 

 informs us that in a full-grown female " the mandibular tooth could scarcely be felt when passing the finger 

 over the gums ; and its existence could scarcely have been ascertained in that way had I not known its 

 position" (P. Z. S. 1876, p. 458). 



