Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/416

312 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 26, Mr. Abbay, the possessor of the magnificent left upper jaw of Megalosaurus now exhibited, to make a definite addition to our means of reconstructing that monstrous Saurian.

The jaw (Plate XII.) is 17.75 inches in length. At its anterior end it measures 4.3 inches in a direction perpendicular to its length. For about an inch and a half from the anterior edge (a), which is entire and shows the natural face of this part of the bone, the upper, or nasal, margin is nearly parallel with the lower, or alveolar, margin; but further back the bone was evidently produced into a great ascending process (b), which divided the nasal from the orbital region. The base of this process is fully four and a half inches long. Its anterior margin slopes rapidly backwards and upwards, and seems to have been nearly straight; while the posterior margin is concave backwards and presents a natural edge, which formed the front boundary of the orbit. The distance in a vertical line from the alveolar margin to the broken upper extremity of the ascending process is 6.75 inches. Behind the ascending process, the vertical diameter of the jaw diminishes, until, at 10 inches from its anterior end, its vertical diameter does not exceed 2.6 in. Behind this point the jaw seems to diminish to a mere bar of bone, not an inch deep at its posterior extremity. But the impression on the lower part of the matrix which occupies the cavity of the orbit shows that the natural edge of the jaw in this region has been somewhat broken away, and that, if it were entire, the depth of the jaw, at 12 inches from the anterior end, would be 2.1 inches, instead of only 1.7 inch as it appears to be. At this place, namely 12 inches from the anterior end, the jaw is transversely fractured; and though the slender prolongation adjusts quite accurately to the broken surface and evidently fitted on, it has, as evidently, lost a good deal along its upper or orbital margin.

Again, the general character of the slender posterior termination of the jaw (c) is such that one would be inclined to think it could not have been directly connected with any other bone; but the part is so much injured that it is not safe to draw any very positive conclusions about the matter. The jaw is traversed by a vertical fracture 2-4 inches from its anterior end. The fracture passes from the alveolar margin to the nasal margin at the commencement of the ascending process. I was at first disposed to think that the fracture coincided with a suture between the praemaxilla and the maxilla; but closer examination does not confirm this supposition, the fracture appearing to be altogether artificial. Hence it would appear either that the praemaxilla and maxilla were so completely ankylosed that all trace of their primitive separation is lost, or that the praemaxilla has become detached from the maxilla, or that the jaw is simply the praemaxilla — a possibility which must not be lost sight of in view of the resemblances between Dinosauria and Birds.

The teeth which remain in their places in the jaw, and are visible from the outer side, are six in number. Five of these, the first, second, third, fourth, and fifth, appear to be completely in place; the third is emerging. On the inner face of the jaw the crown of a sixth tooth, in course of development, lies on the inner side of the base of the fifth.