REPORT ON THE CETACEA. 13 



dentine. This difficulty would have been increased if the kyer situated in the fang 

 between the cement and dentine had been the only one present, as from its position it 

 might have belonged either to the cement or to the dentine. But as a layer of similar 

 structure also existed next the pulp-cavity, there can, I think, be little doubt that it had 

 arisen from the pulp, and notwithstanding the absence of dentine tubes, may be regarded 

 as a modified vaso-dentine, to which substance also the deeper layer of the covering of the 

 fang may be referred. This conclusion is supported also by what is known of the structure 

 of the teeth of some fish in which the dentine consists of a substance destitute of dentine 

 tubes, but possessing a finely-granulated matrix in which vascular canals ramify. ^ 



When I received from Mr Moseley the lower jaw of the adult Mesoplodon layardi, only 

 the left tooth was in its socket, the right had previously been extracted. The socket was 

 situated at the junction of the symphysis with the body of the lower jaw, but more of 

 the tooth was implanted in the body than in the symphysis. The length of the extracted 

 tooth was 14 inches, 6i inches of which had been included in the alveolus, or surrounded 

 by the gum. The breadth of the tooth, where it emerged from the alveolus, was 3i 

 inches. Each tooth consisted of a denticle proper and a strap-shaped shaft. The shaft 

 was laterally compressed, and as it emerged from the socket, it curved obliquely back- 

 wards, upwards, and inwards, so that its inner concave surface had been in relation to the 

 side and dorsum of the beak. As the summit of each tooth passed to the opposite side 

 of the middle line, the two teeth crossed each other on the dorsum of the beak, and from 

 the smooth appearance of the anterior border and inner surface of each shaft it is evident 

 that they must have rubbed against each other, or against the beak, during the movements 

 of the lower jaw in the act of opening the mouth. The shaft represents, though on a 

 much enlarged scale, that part of the young tooth which I have named the fang. 



The denticle proper projected almost directly upwards from the outer edge of the 

 upper end of the strap-shaped shaft, where it became continuous with the anterior border. 

 It was triangular in shape, its base being half an inch, whilst its diameter from apex to 

 base was 3-lOths of an inch (PI. II. fig. 17). The base sprang abruptly from the shaft, and 

 some irregular patches of a glistening white enamel formed its outer surface, but the 

 enamel was not continued upwards to the apex of the tooth, which was formed of dentine. 

 In Professor Owen's figure of the denticle of the tooth of the original specimen, whilst the 

 enamel is apparently worn oft' the tip of the denticle, the base is represented as enveloped 

 by a more complete layer than in this animal. It is also stated that the matrix, by which 

 is obviously meant what I have called the shaft of the tooth, is calcified without 

 enamel. 



In the extracted tooth the alveolar end was seen to be closed, and to terminate, as in 

 Professor Owen's description of the original specimen, in a solid jagged border. The 

 surfoees of the imbedded part of the shaft were grooved with irregular longitudinal 



' Owen, Odontography, 1840-1845. C. S. Tomes, Manual of Dental Anatomy, p. 79, 1876. 



