The skull presents the form of a four-sided pyramid, com pressed laterally, and with the upper facet arching down in a graceful curve to the apex, which is formed by the termination of the muzzle. The very narrow cranium, wide temporal foss on each side, bounded posteriorly by the parietal and the mastoid bones, and laterally by strong compressed zygomata ; the long tympanic pedicle, descending freely and vertically from the point of union of the posterior transverse and zygo matic arches, and terminating in a convex pulley for the articular concavity of the lower jaw ; the large and complete orbits, and the short, compressed, and bent down maxillae, all combine to prove the fossil to belong to the lacertian division of the saurian order. The mode of articulation of the skull with the spine cannot be determined in the present specimen, but the lateral compression and the depth of the skull, the great vertical breadth of the superior maxillary bone, the small relative size of the temporal spaces, the vertical breadth of the lower jaw, prove that it does not belong to a reptile of the batrachian order. The shortness of the muzzle, and its compressed form, equally remove it from the Crocodilians. No Chelonian has the tympanic pedicle so long, so narrow, or so freely suspended to the posterior and lateral angles of the cranium.
The general aspect of the skull differs, however, from that of existing Lacertians, and resembles that of a bird or turtle, which resemblance is increased by the apparent absence of teeth. The dense structure of the produced ends of the pre maxillaries indicates an analogy of function to the tusks of Dicynodon ; the premaxillaries are double, as in crocodiles and Chelonians ; but most of the essential characters of the skull are those of the lizard. The rami of the lower jaw are remark able, as in Bathygnathus, for their great depth, but not the least trace of a tooth is discernible in the alveolar border of the dentary element.
The cranium, in my first described Rhynchosaur, was pre served with the mouth in the naturally closed state, and the upper and lower jaws in close contact. In this state we must suppose that they were originally buried in the sandy matrix which afterwards hardened around them ; and since lizards, owing to the unlimited reproduction of their teeth, do not become edentulous by age, we must conclude that the state in which the RAynchosawrus was buried, with its lower jaw in un disturbed articulation with the head, accorded with its natural condition, while living, so far as the less perishable hard parts of its masticatory organs were concerned. Nevertheless, since
a view of the inner side of the alveolar border of the jaws has not been obtained, we cannot be quite assured of the actual edentulous character of this very singular Saurian. The indica tions of a dental system are much more obscure in the Rhyn chosaurus than in any existing Lacertian ; the dentations of the upper jaw are absolutely feebler than in the chameleon, and no trace of them can be detected in the lower jaw, where they are strongest in the chameleon. The absence of the coronoid process in the Rhynchosaurus, which is conspicuously developed in all existing lizards, corresponds with the unarmed condition of the jaw, and the resemblance of the Rhynchosaurus in this respect to the Chelys ferox, would seem to indicate that the correspondence extended to the toothless condition of the jaws. The resemblance of the mouth to the compressed beak of certain sea-birds, the bending down of the curved and elon gated premaxilhuies, so as to be opposed to the deep symphysial extremity of the lower jaw, are further indications that the ancient Rhynchosaur may have had its jaws encased by a bony sheath, as in birds and turtles, the dentinal ends of the pre maxillaries projecting from, or forming, the deflected end of the upper mandible.
There are few genera of extinct reptiles of which it is more desirable to obtain the means of determining the precise modifications of the locomotive extremities than the Rhyncho saurus. The fortunate preservation of the skull has brought to light modifications of the lacertine structure leading towards Chelonia and birds which before were unknown ; the vertebrae likewise exhibit very interesting deviations from the lacertian type. The entire reconstruction of the skeleton of the Rhyncho saurus may be ultimately accomplished, if due interest be taken in the collection and preservation of the fossils of the Grinsill quarries.
The cranium of a Rhynchosaurian reptile with small palatal teeth and obscure maxillary dentations, has been discovered in the problematical sandstones, containing the Leptopleuron, near Elgin ; and adds to the probability of their triassic age.