Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/516

 where, at 7.5 inches from the postarticular ridge, it makes a coronoid eminence, which is less elevated than that of lizards, but which rises more than that of crocodiles. From this onwards the border sinks at first in a rather steep curve, and then runs forwards nearly horizontally to the straight dentary part.

The lower border of the ramus sweeps downwards and forwards from the posterior extremity in a nearly uniform curve to about 6 inches behind the hindermost tooth, from which point onwards it is nearly straight. Six inches from the posterior extremity, and 4 inches behind the coronoid process, the curve is broken by a distinct angle.

The form of the toothed part may be roughly compared to a three- sided prism. The upper surface is nearly flat, and its inner border rises slightly higher than its outer border, which is pierced by the alveoli at short intervals. The inner surface is vertical. It is scored by furrows and striae, which begin a little distance in front of the angle which the straight part makes with the curved part of the jaw, and which mark, I think, the beginning of the symphysial suture. The outer and under surface, much defaced, is transversely convex. Behind the straight dentary part, the ramus loses in thickness, and at about 6 inches from the last visible tooth it begins to gain in depth, while the inner and outer surfaces becoming parallel, the transverse section has here an oblong figure. From this backwards the depth of the ramus increases till it attains its maximum in the coronoid eminence, behind which it declines to the posterior extremity.

The large vacuity in the outer surface, between the angular, surangular, and dentary bone, present in all living crocodiles, and, as far as I can learn, in all extinct ones of the Tertiary period, is entirely absent from this Kimmeridge jaw, in which the component bones meet closely. For about 16 inches from the posterior extremity, the outer surface is nearly equally shared by the angular bone below, and an upper splint comprising, I suppose, the dentary and surangular, with a small portion of the articular bone. About 1-1/2 inch in front of the postarticular ridge, and therefore just at the front of the articular surface, on the upper border, there is a fissure filled with stone, which may be the suture between the articular and surangular bones ; but I cannot trace any suture marking the respective shares of the surangular and dentary bones, unless a fissure running along the bottom of a groove which indents the surface of the jaw, parallel with its upper border, for 7 inches forwards from the coronoid eminence, be part of it, of which I feel very doubtful. This composite surangular and dentary splint, with the articular, forms the upper border and the greatest part of the posterior extremity of the ramus. From this latter to a short distance in front of the articulation, the natural surface of the bone has suffered from chipping, and is quite destroyed here ; but it reappears in front of the articulation, and it exhibits, for about 12 inches, a fine longitudinal striation ; the striae spread from a point in the lower border of the splint, below the articulation, in a fan-