426 PROF. W. F. FLOWER ON THE GENUS MESOPLODON. 
death can scarcely be determined now; but Dr. v. Haast mentions that in his recent 
specimens some of the teeth were “ evidently worn down from use.” The crowns of 
the largest of these teeth are 4 millims. in length, projecting that distance above the 
gum in its present dried state, though when recent they must have been covered toa 
larger extent. Some teeth of an older individual (figured in P. Z. S. 1876, p. 10) are 
somewhat larger. Their whole length, including crown and root, is 12 millims. or 
half an inch, and their greatest thickness (in the part concealed by the gum) 2} millims. 
There is no evidence of the presence of any such teeth in WV. australis or M. hectori; 
and whether they were present or not in WM. haasti I am not able to say, as the bones 
were cleaned before they came into my possession. 
The genus Mesoplodon is characterized by possessing a single pair of compressed 
mandibular teeth, which are usually situated at about one third of the distance from 
the apex of the mandible to the condyles, or opposite the hinder edge of the symphysis. 
This is, in fact, their situation in all known species, except I. europeus and M. hectori; 
and it is doubtful whether the first should be considered an exception; for though the 
teeth are situated further forward than in the others, relatively to the whole ramus of 
the mandible, they have the same relation to the hinder border of the symphysis, which 
appears (judging only by the published figure) to be shorter than in the other species. 
The teeth of the different species appear to agree in their essential structure’, having 
a small and pointed enamel-covered crown, composed of true dentine, which, instead of 
surmounting a root of the ordinary character, is raised upon a solid mass of osteo- 
dentine, the continuous growth of which greatly alters the form and general appear- 
ance of the organ as age advances, as seen most strikingly in the case of W. layardi. 
The changes that take place during growth in the mandibular teeth of IZ. grayi are 
well exemplified in the different specimens described by Dr. v. Haast. Inthe young 
male skeleton the apex of the tooth projects but a quarter of an inch above the level 
of the alveolar margin, and is quite unworn. During life it was probably altogether 
concealed beneath the gum, though the animal had attained three fourths of its 
full dimensions. The apex is directed upwards and outwards. It lies in a special 
expansion of the dental groove, which groove, it should be mentioned, continues to 
exist, even in the aged animal, in a marked degree, along the whole upper border of 
the dentary portion of the ramus. Although quite loose, it could not be removed to 
examine its base until a portion of the bone had been cut away; yet in the old tooth, 
which completely filled the socket, owing to the alteration of the form of the latter 
this difficulty did not exist. 
When removed the tooth has a triangular form; the apex, which is pointed and 
somwhat everted, is covered with a thin layer of enamel, which extends to a greater 
distance (¢.¢. 13 millims.) in front and behind than at the sides (7 millims.) The 
* See E. Ray Lankester, Trans. Microse. Soc., new series, vol. xv. 1867, p. 55, and Trans. Zool. Soe. vol. viii. 
P 
p. 223, 
