20 THE VOYAGE OF H.M.S. CHALLENGEE. 



acquired greater opacity from the increase and general distribution through it of minute 

 interglobular spaces, with which the line of interglobular spaces abeady described in the 

 upper part of the tooth became continuous. But there also appeared between the dentine 

 and cement a definite layer, at first thinner than the cement, but increasing in thickness as 

 it extended down the fang, in the lower part of which it equalled in thickness the cement 

 and dentine together (fig. 22). This layer was readily recognisable to the naked eye from 

 its opaque white appearance. It contained numerous branching and anastomosing canals, 

 the chief of which lay perpendicularly to the surface of the tooth. The matrix between 

 the canals was granulated. This layer corresponded, therefore, in structure to the modified 

 vaso-dentine described in the teeth of Mesoplodon layardi. 



To the naked eye the wall of the pulp-cavity had numerous hemispherical bodies 

 projecting from its free surface. When examined microscopically they were seen to be 

 continuous with the dentine, for the dentine tubes were prolonged into them. The 

 dentine formed, therefore, the wall of the pulp-cavity in the greater part of its extent ; but 

 the wall of the constricted part of the cavity in proximity to the end of the fang, and at 

 the sides of the chink-like opening in it, was not dentine, but consisted of the substance 

 which I have named modified vaso-dentine. It was not, however, so regularly constructed 

 as the layer between the dentine and cement, for the canals were few in number in pro- 

 portion to the matrix, and had no definite arrangement. 



I shall now make some observations on the leading differences between the tooth of this 

 young Mesoj^lodon sowerbyi, and that of the adult animal described by Professor Lankester. 

 In the first place, the crown of the tooth of the adult projected (as I have ascertained 

 from a measurement of a cast of the jaw presented by Dr Acland to the Anatomical 

 Museum of the University of Edinburgh) 1 j^ths beyond the edge of the alveolus, whilst 

 only the tip of the tooth in the young animal projected out of the socket. The outer 

 surface of the young tooth was almost uniformly smooth, and not rough and knotted as 

 in the adult. The pulp-cavity, instead of being almost equal to the entire length of the 

 tooth, was restricted in the adult to a small space in the crown, the rest of the tooth being 

 solid. In this respect the tooth of Mesoplodon soiverhyi approximates to what I have 

 described in the shaft of the tooth of the adult Mesoplodon layardi. The early stage of 

 the closing up of the pulp-cavity is to be seen even in the young Mesoplodon sowerbyi, in 

 which almost the whole of the cleft at the root of the fang is closed up, and the walls of 

 the adjacent part of the pulp-cavity are closely approximated to each other. The enamel 

 had evidently been worn ofi" the crown of the adult, for Mr Lankester makes no reference to 

 it. The dentine in the adult was confined to a small conical cap at the apex of the crown, 

 and to a very thin layer extending about half-way down the tooth, instead of, as in the 

 younger tooth, forming the larger proportion of its substance. The great bulk of the 

 adult tooth was made up of cement, osteo-dentine, and of a substance which Mr Lankester 

 calls globular matter. The cement was evidently considerably thicker in the adult than in 



