The free caudal vertebrae are generally amphicoelous. THE SKULL. The skull of all birds from _Archaeopteryx_ onwards is essentially similar, differing from the skull of reptiles mainly in the extent to which the cranium is arched, and its greater size in proportion to the jaws. Most of the bones of the cranium are pneumatic, and all show a marked tendency to fuse together, and have their outlines obliterated by the disappearance of the sutures. The several bones remain longest distinguishable in the Ratitae and to a less extent in the Penguins. The orbits are very large and lie almost entirely in front of the cranium; they are separated by an interorbital septum which is sometimes, as in _Chauna_ and _Scythrops_, very complete, sometimes, as in Hornbills and the Common Heron, very slightly developed. As a general rule the sclerotic is cartilaginous. The anterior nares are almost always situated far back at the base of the beak near the orbits, but in _Apteryx_ they are placed right at its extremity. In _Phororhacos_ they are placed very high up on the enormous beak and are not separated by any bony partition. The skull of Parrots has some peculiarities. In some Parrots the lachrymal sends back a process which meets the postorbital process of the frontal and completes the orbit. In most birds the upper beak is immovably fixed, but in some it is attached to the cranium, only by the nasals and by flexible processes of the premaxillae, so that by this means a kind of elastic joint is established and the beak is able to be moved on the cranium. In the Parrots and _Opisthocomus_ there is a regular highly movable joint. In Cassowaries the fronto-nasal region of the skull is produced into an enormous bony crest, and in Hornbills a somewhat similar structure occurs. Although true teeth do not occur in any known bird except _Archaeopteryx_, _Hesperornis_, and _Ichthyornis_, another extinct bird, _Odontopteryx_, has the margins of both jaws provided with forwardly-directed tooth-like serrations, formed of part of the actual jawbone: a living hawk, _Harpagus_, too, has a deeply notched bill, to which correspond serrations in the premaxillae. A basi-pterygoid process of the basisphenoid abuts against the pterygoid in Ratitae and in Tinamous, plovers, fowls, pigeons, ducks and geese among Carinatae, recalling the arrangement met with in many reptiles. The squamosal is sometimes, as in the fowl, united with the postorbital process of the frontal. In the Carinatae the quadrate articulates with the cranium by a double convex surface, in the Ratitae by a single one. The premaxillae are always comparatively large bones, the maxillae on the contrary are small, but give rise to important inwardly-projecting maxillo-palatine processes.