The supraoccipital has a distinct median ridge, with a longitudinal depression on each side, bounded externally by a prominent convexity. ([Pl. 10], fig. 1.)
MANDIBLE.
The mandible is slender, with a very elongate symphysis, which measures 237 mm. The inferior outline of the ramus is strongly concave at the middle and slightly convex posteriorly, while the symphysial portion is bent upward. The superior outline is concave both behind and before the tooth, and also immediately anterior to the coronoid process. At about the beginning of the posterior fourth the outline is convex, and the mandible at this point is nearly as deep as at the coronoid process. The superior surface of the symphysis slopes down on each side to the median line, but each half of the surface is itself nearly plane. ([Pl. 11], figs. 1, 2, and 5.)
The alveolar groove anterior to the tooth is very distinct throughout and is without septa and open at the bottom. It ends 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]