10 THE VOYAGE OF H.M.S. CHALLENGER. 



He has most courteously placed his notes at my disposal, and in this place I shall c[uote 

 what he says respecting the lower jaw. 



" The peculiar upward curve of the inferior edges of the rami of the mandible immedi- 

 ately behind the termination of the symphysis is alike in both specimens. The upper sur- 

 face of the symphysial portion is in the New Zealand specimen broader than in the one 

 from the Cape ; the whole beak-like mass constituting this symphysial portion being less 

 attenuated and not presenting such sharp projecting alveolar ridges. In the New Zealand 

 specimen the symphysial line appears superiorly as a deeply-sunken groove, to the suture 

 at the bottom of which the bone curves evenly downwards on each side, whereas in the 

 Cape specimen a slightly raised ridge in each bone runs parallel and close to the suture. 



"In the New Zealand specimen the under surface of the symphysial beak is evenly 

 rounded, except at its very hindermost part, where two very slight ridges are to be made 

 out, situated one on each side of the symphysial line, and passing ofi' into the inferior 

 edges of the two rami. In the Cape specimen the symphysial beak shows a lateral groove 

 towards the tip, situated below the alveolar border, and instead of being rounded on its 

 under surface, is compressed and sharply keeled along the middle line. The mental 

 foramen in the outer surface of the jaw below the position of the tusk is a small and 

 simple aperture in the New Zealand jaw, but in the one from the Cape it is large, and 

 leads to a canal in the bone. The swelling caused by the large alveoli of the tusks is alike 

 in both specimens. The portion of the symphysial beak in front of the spot where the 

 anterior margin of the tooth emerges from the alveolus is about 1 inch longer in the Cape 

 specimen than in that from New Zealand. In the New Zealand specimen the alveolar 

 margin of the rami behind the teeth is very much compressed and sharp, and the 

 alveolar groove can only be traced for about 2 inches backward : it then becomes 

 obliterated when the margin of the jaw is knife-edged. In the Cape specimen the 

 alveolar margin is much broader, and the groove visible up to the cut end of the rami." 



In the younger specimen C the alveolar groove extended from the anterior end of the 

 mandible to 7\ inches behind the socket for the tooth. It was a narrow groove in its 

 entire extent except in the region of the tooth, where it expanded into an appropriate 

 socket. A weU-marked mental foramen communicated with the inferior dental canal below 

 the position of the alveolus. 



The Teeth. — I shall now proceed to describe the form and structure of the very 

 remarkable teeth of Mesoj^lodon layardi, and as the study of the tooth in the younger 

 animal has thrown great light not only on the structure but on the peculiarities of form 

 of the adult teeth, I shall in the first instance describe it. 



The teeth of the young Mesoplodon layardi were imbedded in their sockets, one in 

 each half of the lower jaw. Each tooth consisted of a small triangular denticle, which 

 represented the crown of the tooth, and of a larger part, which for descriptive purposes 

 may be termed the fang. The denticle projected outwards and slightly upwards from 



