EEPORT ON THE CETACEA. 21 



my specimen, and the osteo- dentine and globular matter together formed a large proportion 

 of the adult tooth. In the younger tooth, well-marked vaso-dentine was present, as already 

 described, but I could not say that I recognised any definite osteo-dentine. The material 

 which I have named modified vaso-dentine was also present in considerable quantity, and 

 in its opacity it corresponded with the globular matter of Prof. Lankester. In its structure, 

 however, it appears to differ, for he describes the globular matter as having " no structure 

 excepting an indistinct botryoidal character visible with a low magnifying power." " The 

 amorphous matter at length shades ©ff" into tlie dentine, numerous distinct, minute, ' inter- 

 globular spaces ' becoming more and more distinct as one recedes from the opaque stratum, 

 and their number diminishes." It is probable that this globular matter may represent in 

 the adult the modified vaso-dentine of the younger tooth, for the numerous vascular canals 

 which the latter contains may become obliterated through an extension of the process of 

 calcification, so as to give it the more solid character present in the fully-formed tooth. 

 In the granulated matrix of the younger tooth, an appearance was not unfrequently seen, 

 which might have been described as interglobular spaces. 



From Professor Flower's description of the structure of the teeth of Berardius 

 arnouxii,^ it would appear that in that ziphioid the teeth are very similar to those of the 

 adult Mesoplodon soiverhyi described by Prof. Ray Lankester. 



The observations which I have now recorded on the non-erupted teeth, both of 

 Mesoiolodon layardi and Mesoplodon sowerhyi, prove, that in the earlier stages their 

 structure does not diff'er materially from the ordinary type of tooth one meets with, say 

 in the human or carnivorous jaw, the crown being covered by enamel, the fang by 

 cement, whilst the great body of the tooth consists of dentine, in which is a well-marked 

 pulp-cavity, communicating with the exterior by a slit-like aperture at the root of the 

 fang. The exceptional character which these teeth exhibit in the erupted condition is 

 due to the disappearance of the enamel from the crown, to the cessation in the develop- 

 ment of the ordinary dentine, to the excessive formation of osteo-dentine, of modified 

 vaso-dentine, and of cement, by means of which the pulp-cavity becomes almost obli- 

 terated, and the fang assumes dimensions which, in the case of Mesoplodon layardi, lead 

 to the production of a tooth having the very remarkable form and relation to the beak 

 which I have described.^ 



I shall next describe the other bones of the axial skeleton of the younger Mesoplodon 

 layardi (specimen C), which consisted of the spinal column, ribs, sternum, and a portion 

 of the hyoid bone. 



Spiiwbl column. — The length of the column in the macerated spine was, with the 



^ Trans. Zool. Soc, vol. viii. p. 223. 



^ I have not thought it necessary to figure the skull of the adult Mesoplodon layardi, as the illustrations given by 

 Professor Owen in his Monograph on the British Fossil Cetacea in the Memoirs of the Palseontographical Society, 1878, 

 express so well the characters of the adult skull. As the petrous bone, however, of the adult has not been figured, and 

 as so immature a skull as that described in the text has not previously been examined, I have had them drawn in Plate I. 



