1901.] 



ANATOMY OF PICAKTAN BIHBS. 



19 



differs is in the form of the occipital condyle. In B. caftr it is 

 a little more elongated transversely than in B. abi/ssinicvs. 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 bo distinguished. 



Characteristics of the Skull of Bucorvus. — These can be arri\ed 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, 

 iu 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, § • ( X -g.) 



which is formed of finely and beautifully cancellated bone. In 

 Bucorvus, en 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 shghtjy projecting marks the an- 

 terior 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-case 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 con- 

 spicuously 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 



9* 



