distally in a rounded aperture 6 mm. in diameter, below which are several small foramina. These lead to a very large canal which occupies all the symphysial portion of the mandible, the walls being comparatively thin. Behind the tooth the alveolar groove becomes narrower gradually and disappears in a length of about 140 mm.

The mental foramen is situated in line with the anterior base of the tooth, and is confluent with a groove which extends forward for about 80 mm. A rather shallow groove runs along the inferior margin of the symphysis.

The coronoid process is erect and rounded, and is joined by a horizontal ridge anteriorly.

TEETH.

The mandibular tooth, which is shown in Pl. 2, fig. 3, is preserved on the right side only. Its dimensions are as follows: Length anteriorly in a straight line, 75 mm.; length from the apex to the posterior end of the root, straight, 60; greatest antero-posterior breadth, 28; transverse thickness, 10; height of apex above internal superior margin of jaw when tooth is in situ,[13] 22; antero-posterior length of base of exposed portion, 30; distance from anterior end to posterior end of root, 37; greatest height of the exposed dentine crown, above the cement, 14; length of the base of the dentine crown, 12.

This tooth, as already stated, is only two-thirds as broad and three-fourths as long as that of Sowerby’s Brodie House specimen (the type of the species), which was an adult male, and leads to the belief that the Nantucket specimen was a female. This is in a manner confirmed by the Rugsund specimen, which was an adult male and had teeth as large as Sowerby’s specimen. It has to be remarked, however, that in the Overstrand, England, specimen (1892), which was an adult female, the teeth did not project beyond the gums. Messrs. Southwell and Harmer say regarding it:

The jaws were apparently completely edentulous, and although it was possible to feel through the gums a slight prominence on either side in the position of the teeth of the male, we could not by this means definitely satisfy ourselves with respect to this point, nor were we able to ascertain the presence of any other rudimentary teeth in either jaw. The evidence which exists on this subject is favourable to the view that the female of this species is not provided with any teeth which are large enough to pierce the gums.[14]

It is probable that the teeth in the Nantucket specimen, though quite large, did not project beyond the gums any considerable distance. The external border of the alveolar groove behind the tooth is only 20 mm. below the apex of the tooth, and it is not unlikely that the gums in a specimen of this size had nearly that thickness, so that only the tip of the tooth would project beyond them. Though the apex is acute, it has a flat abraded surface anteriorly, which, however, is but 4 mm. long. It seems probable, on the whole, that the teeth in the female may be quite large without projecting more than a few millimeters beyond the gums.

In shape the tooth of the Nantucket specimen is almost identical with that of Sowerby’s Brodie House adult male, as figured by Lankester. The dentine at the apex is more nearly white than the cement which surrounds it. The superior margin of the latter is not a plain ring, but sends upward a papilliform projection on each side. The dentine itself has two vertical grooves on each side. The root of the tooth ends very obliquely and is rugose and irregular. The cavity is closed.

Grieg remarks as follows regarding the structure of the teeth of the Rugsund specimen:

Sections and microscopic preparations of the alveolar tooth of this whale show that its apex consists of dentine, within which is found an inner pulp cavity 4 mm. long and 1 mm. broad. The dentine, the structure of which agrees with that which Turner found in Mesoplodon bidens and Mesoplodon layardi, is yellowish white, with the exception of the part nearest the pulp cavity, which is yellowish brown. It seems to correspond most closely to what Ray Lankester called osteodentine. Throughout the tooth the dentine is covered with a very thin layer of shining white enamel. The enamel is, however, lacking on the front of the tooth, having probably been worn away. A section through the middle of the tooth, at right angles with the V-shaped furrow, shows a yellowish cement layer from 3 to 5 mm. broad, which is, however, worn away on the front of the tooth. Within the cement layer is a white, amorphous, calcareous mass, forming a band from 1.5 to 3.5 mm. broad, which appears to correspond to Ray Lankester’s “globular matter” and Turner’s “modified vasodentine.” The mass seems to agree most closely with Ray Lankester’s “globular matter,” as it has “no structure excepting an indistinct botryoidal character visible with a low magnifying power.” The core of the tooth consists of dentine, the inner layer of which is brownish, while the outer is rather whitish yellow. As above mentioned, the dentine is visible on the front of the tooth, since both the cement and the amorphous, calcareous mass are worn away. Moreover, it is clear that on the front of the tooth the dentine is not covered by enamel. The pulp cavity is reduced to a fine pore. A section across the root of the tooth shows an outer yellowish cement layer, from 2 to 5 mm. broad, while the interior of the tooth is filled with a white, amorphous, calcareous mass, which is interspersed with thin yellowish lamellæ of dentine. Here and there, also, thin lamellæ are seen to extend from the outer cement layer into the white, amorphous, calcareous mass. The dentine lamellæ appear to be identical with what Ray Lankester calls osteodentine. No pulp cavity is visible in the root of the tooth.[15]

The dimensions of the Nantucket skull are given in the following table in comparison with those of seven European skulls of M. bidens. Dimensions of the Annisquam, Massachusetts, skull are also added for purposes of comparison, although it represents another species (see p. 9).

Dimensions of eight skulls of Mesoplodon bidens and one skull of M. densirostris (?).

Column headings:

M. bidens.

B: Nantucket, Massachusetts, 1867, M.C.Z., female? adult.^a

C: Scotland, 1872, Turner, female young.?

D: Fæø, Norway, 1895, Grieg, female? young.

E: Shetland, 1881, Turner, male adult.

F: Rugsund, Norway, 1901, Grieg, male adult.

G: Udsire, Norway, 1869, Malm, male (No. 1).

H: Vanholmen, Sweden, 1881, Malm, male (No. 2).

I: Landenæs, Norway, 1895, Grieg, male.

M. densirostris. (?)

J: Annisquam, Massachusetts, 1898, True, female young.