OCR Text |
Show 20 MR. F. E. BEDDARD ON TnE [Jan. 15, of bone to form a projecting ridge which joins the jugal arch behind. This renders the margins of the orbit perfectly visible when the skull is viewed directly from the front. In Buceros there is no such ridee, and the orbit is invisible when the skull is looked at from in front. The narial aperture is double on each side in Bucorvus as it is in, for example, the Toucans. Each of the two apertures into which the originally single aperture is divided in this genus is of a rather elongated oval outline. In Buceros the single narial orifice is circular in outline. A comparison of the dorsal aspect of the skull in the two genera shows several points of divergence in the two types. The greater breadth of the cranium of Bucorvus is apparent, this being mainly due to the projecting shelf of bone over the orbit, already referred to. Furthermore the " lacrymal " ring in front of the orbit which is absent or at least not so fully developed in Buceros, causes a very sharp demarcation between the cranium and the face in Bucorvus, a distinction which is wanting in Buceros, where one region gradually fades into the other. The commencement of the beak region is quite as wide as the anterior part of the orbit in Buceros ; in Bucorvus it is very plainly much narrower. The contrast is so great that measurements are unnecessary to express the differences. The basal aspect of the skull of Bucorvus is in some respects different from that of Buceros. In the first place, the foramen magnum in Bucorvus is much more distinctly upon the ventral surface than in Buceros, where this foramen looks partly backwards. It thus happens that the dorsal wall of the foramen is more apparent on a dorsal view in Buceros than it is in Bucorvus. The palatal region too shows differences which are not without a certain interest in relation to the connection between the two types of Hornbill. As has been already recorded by Fiirbringer, the Bucerotidae possess basipterygoid processes. These are, however, rudimentary, and are far from being in contact with the pterygoids. In Buceros not only are there present a pair of somewhat jagged rudimentary basipterygoid processes, but the pterygoids themselves are bowed inwards opposite to these processes ; at the place where they should, so to speak, articulate with the basipterygoid processes they bear a roughened outgrowth which seems to suggest the remains of a pterygoid articular facet. So exactly does the position of this facet correspond to the basipterygoid process, that if the bones eould be forciblv pushed togerher they would meet at those points. Bucorvus shows a further stage of degeneration, which fits in well with the presumption that it is a later type than Buceros. The basiptervgoid processes are distinctly more rudimentary, and, indeed, they are only just recognizable in B. abyssinicus. The pterygoids are straight, and are not at all bowed inwards towards the basipterygoid processes. In the place of what I have regarded as an articular facet upon the pterygoids in Buceros, there, is in Bucorvus a thin, large, upwardly directed lamellar process of bone. This, however. |