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Show i^OL] ANATOMY OF PIC'ARIAN BIRDS. 19 differs is in the form of the occipital condyle. In B. cafer it is a little more elongated transversely than in B. abyssinicus. As will be seen from an inspection of the accompanying drawings (text-figs. 2, 3), the outline of the orbit is a little different iu the two species. In other respects the two skulls can hardly be distinguished. Characteristics of the Skull of Bucorvus.-These can be arrived at from a comparison of the two species of Bucorvus with a skull of Buceros rhinoceros, which I shall take as a type of the arboreal Hornbills, indicating at the same time such divergences as are exhibited by other arboreal Hornbills. In comparing the skulls of the two, the first striking difference between the two genera is that shown by the cancellated bone which fills the casque. This, in Buceros, is solid behind where it projects back considerably over the roof of the skull; anteriorly it ends abruptly in a steep declivity Text-fig. 3. Skull of Bucorvus cafer, $. (xi) which is formed of finely and beautifully cancellated bone. In Bucorvus, on the contrary, whether the cancellated bone shows exteriorly or not, the whole bony process slopes gradually, first upwards and then downwards in an even curve, there being no abrupt demarcation between it and the maxillae in front. In Buceros a delicate shelf of bone slightly projecting marks the anterior boundary of the bony part of the casque. When the skulls of the two Hornbills are viewed laterally, notable differences are obvious. The walls of the brain-ease are seen to arise in Buceros to a considerable distance above the orbit. The top of the skull is in fact swollen and convex. In Bucorvus, on the other hand, the top of the skull is almost flat and it is continued to form a projecting shelf over the orbit, which thus stands out more conspicuously from the sides of the head than in Buceros. The prominence of the orbit in Bucorvus is further emphasized by the prolongation downwards in front of the lacrymal region of a plate 2* |
