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. THE DENTARY BONE Pl. 12. The dentary bone consists of a single piece, as in birds and chelonians; and differs from both in being provided with teeth. It is described under the species O. machærorhynchus, page 113. ------------ THE TEETH. Pl. 12. Case. Comp. Tablet. Specimen. =J= _c_ 17 1-39