Transactions. — Zoology. 



Art. XY. — Notice of a Variation in the Dentition of Mesoplodon liectori, 

 Gray. By James Hector, M.D., F.R.S. 



Plate XY.A. 

 [Read before the Wellington Philoso2?hical Society, 13t7i October, 1873.] 

 This specimen is only a fragment of the lower jaw, but the portions preserved 

 afford sufficient characters to determine the genus, and to show that it differs 

 from any described specimen, and also throw some light on the little known 

 subject of the dentition of the Ziphiidce. 



The posterior portion of the left ramus from the condyle to the symphysis, 

 and the middle portion of the right ramus, is all that remains. About two 

 inches of the surface of the suture enable the angle of divergence of the jaws 

 to be determined, and, therefore, the width of the cranial articulation. 



Opposite the commencement of the symphysis (sy) was a deej)ly excavated 

 alveolus, which, in the left side, contains a triangular tooth (t). The dental groove 

 has evidently been open and deeply excavated in the back part of the jaw, and 

 where broken off on both sides, it appears as if it was expanding to form a 

 second tooth chamber [t'). 



Left Ramus. — The condyle is very feeble, but not more so than in 

 M. hectori, to which the jaw bone is very similar except as regards the 

 teeth.* The articular surface is 1'3 inches long and 0-3 inch wide, and 

 situated above the middle of the posterior border, which is convex externally 

 and 4 inches in length. On the inside the posterior third of the bone is 

 deeply excavated, very thin, and on the outer surface shows one large 

 inferior and a small superior ridge. The upper margin is compressed, thin, 

 and elevated to form a blant angle, behind which the dental groove begins at 

 6 inches from the condyle. The inferior margin is rounded, but with a blunt 

 keel-like ridge, and slopes up rapidly, reducing the width of the ramus from 

 3*3 inches posteriorly to 1-5 inches in its middle third. This middle third is 

 solid, compressed, and obliquely turned like a ploughshare, so that at the 

 symphysis the dental grooves are on the outer side of the jaws 2 inches apart, 

 and separated by a flat area formed by the symphysial processes, which are 

 conjoined to form the terminal portion of the lower beak. The width at the 

 condyles was probably 8 inches ; length to the symphysis, 12-5 inches. 



Tooth. — The chief interest of this specimen is the dentition, as in this it differs 

 from the type specimen both in the position and character of the tooth. In the 

 first respect it resembles the original type of the genus M. sowerhyensis, as the 

 tip of the tooth {t) is exactly opposite to the posterior end of the symphysis {sy). 

 Behind it the dental groove, though distinctly visible, is quite closed, but it 



* Trans. N.Z. Inst, Vol. II., p. 27 ; Vol. III., p. 125 ; Vol. V., p. 167- 



