Page:Quarterly Journal of the Geological Society of London, vol. 34.djvu/479

Rh THE CORALLIAN ROCKS OF WEYMOUTH. 399

having been accomplished, the trouble expended upon it was repaid by the peculiar characters of its front end, which were thereby displayed. The fracture and displacement of the right ramus is shown also in this view (fig. 2). The superior size of the 2nd, 3rd, 4th, and 5th alveoli is well seen, as well as the remains of teeth which several of them contain. In order, apparently, to accommodate these larger teeth, the jaw is wider in this region than in that which is immediately behind it, where the alveoli are smaller. On each side, just within the large alveoli, there is a deep groove, extending from the front backwards to the fifth or sixth tooth. The inner edge of the alveolar border in this region is on a level with the median area of the jaw, as shown by a transverse section, figure 3, while its outer edge is depressed, so that the general plane of this border forms an angle of about 15° with that of the median area. Behind the fifth tooth, the alveolar border, while retaining the same general direction, becomes rapidly lowered until its inner edge is on a level with the bottom of the groove, so that the groove having lost one of its sides, be- comes a kind of step, and a transverse section in this region has the appearance shown in figure 4. The median area is almost flat, each half, however, being slightly rounded. The most remarkable pecu- liarity of this jaw is the manner in which about two inches of the anterior part of the median area becomes separated from the hinder portion, in the form of a spindle, by deep oblique grooves. These grooves pass from the lateral ones opposite the third pair of teeth, and meeting in the middle line dip down into the substance of the jaw or, rather, into a channel which appears to have existed in the symphysis. The spindle-shaped area is longitudinally grooved; whilst the area immediately behind the oblique grooves, although somewhat broken, gives evidence of having possessed a longitudinal ridge close to the symphysial suture.

The teeth are fragmentary ; but still, from what remains of them, several of their characters may be made out. Allusion has already been made to the fact that the 2nd, 3rd, 4th, and 5th teeth upon each side are larger than the others. In two or three instances the young teeth may be seen within the broken roots of the mature ones. The bases of the largest teeth must have had a diameter of at least half an inch ; and, judging from an impression in the matrix, they projected about an inch and a quarter above the margin of the jaw. This impression also shows that there was no very marked division between the crown and the fang of the tooth, the former, however, being distinguished by the possession of fine, but very distinct longitudinal ridges. In transverse section the greater part of the tooth appears to have been circular ; but the apex of a young unused tooth which is well preserved in the third alveolus of the left ramus is slightly compressed, having a distinct ridge running down each side, and two smaller ridges on the inner face ; a transverse section near the apex would give an outline as in figure 5 b.

With regard to the length of the symphysis, one cannot speak with certainty, on account of the parts being displaced ; but while it seems extremely probable that it extended at least as far as the