Art. XV.—Notice of a Variation in the Dentition of Mesoplodon hectori, Gray.
[Read before the Wellington Philosophical Society, 13th 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 ZiphiidŒ.
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 deeply 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 blunt 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. sowerbyensis, 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
[Footnote] * Trans. N.Z. Inst., Vol. II., p. 27; Vol. III., p. 125; Vol. V., p. 167.
dilates to form a dental cavity, in which the tooth is lodged, its tip onlyreaching to the level of the upper edge of the jaw. It occupied the cavity loosely, but could not be removed without breaking the bone. The tooth (figs. 3 and 4) is a very thin, hollow, conical shell, compressed, but not filled with solid dentine as in previous specimens. The tip is smooth and enamelled. The height is 1.2 inches; width, 1.0; and its thickness, 0.3.
The tooth shows the specimen to have been a young animal, and this will account for the slightly smaller size and different proportions of the jaw from the type; and the posterior, instead of the terminal, position of the tooth shows that probably several existed on each side, and that the dental characters in this group of whales are not constant or sufficiently important to form the basis of specific distinction. At the same time it is interesting to find that, even in the young state, this whale has the compressed form of teeth and the same mode of their arrangement which obtains in Berardius, to which genus it was at first referred. *
This specimen was found on the beach at Kaikoura, and forwarded to the Museum by Mr. J. R. W. Taylor,
[Footnote] * Trans. N.Z. Inst., Vol. III., p. 108.