This determination is conjectural. Its form is such as would make it probable that it is part of the head. A more perfect specimen is seen in J.c.1.2.7.
The best specimen is a compressed sub-quadrate fragment of bone terminating at one end in a long reniform articular surface, and at the other end in a fracture where the bone is rapidly thickening. A side, regarded as the outer one, is flattened, being slightly concave in length, and slightly convex from side to side. The form of the inner side of the bone is determined by the inward curve of the thick part of the articular surface, which sends a rounded ridge obliquely on to the side, so that while it is concave from side to side at the articulation, at the fracture it is convex from side to side. All the specimens are large, the articulation being not less than an inch long.
PREMAXILLARY BONES
[Pl. 12.]
appear to be developed as in birds. An account of their structure will be found in the notes on the species, [page 112].
| Case. | Comp. | Tablet. | Specimen. |
| J | c | 12 | 1—6 |
OS ARTICULARE AND PROXIMAL
END OF LOWER JAW.
[Pl. 12.]
Prof. Owen has described in a 'Palæontographical' monograph the proximal end of a mandible in which the sutures are obliterated. But there is one specimen of a young right ramus showing the inner and under part of the mandible to be the surangular bone which unites with the angular or outer bone by a longitudinal and vertical suture traversing on the inner side the great upper groove; and on the surangular the greater part of the articular bone rests. The articulation is strong and double, consisting of a deep transverse hollow, bounded by a strong over-locking ridge in front and a slight ridge behind; and this area is divided into two tapering furrows by a strong oblique and rounded crest, which passes from behind inward and forward. Just behind the articulation is a ?pneumatic aperture, and then the upper surface tapers to the under surface, forming a heel, of which one specimen measuring an inch deep on the inside of the articulation has 3/4 of an inch still left and is more than 1/4 inch thick at the fracture. In a specimen belonging to the Rev. T. G. Bonney the outside of the jaw is 11/16 of an inch deep, and under the articulation 5/16 of an inch deep. The articular area is 3/4 of an inch wide and 6/16 of an inch long.
Seven specimens indicate four species.
The proximal end of the lower jaw is entirely Avian. The pneumatic aperture, as in birds, is placed behind the articulation, which is shaped as in many birds. Commonly in Ornithosaurians the bones are anchylosed and all trace of sutures obliterated, as in most birds. In the Goose, however, the six elements of each side are sometimes as readily separated as in reptiles. And in some Pterodactyles the bones separate.