OCR Text |
Show 1903.] OSTEOLOGY OF THE CUCULIFORMES. 265 the metencephalic fossa, midway between the sella turcica and the floccular fossa, and is overhung by a sharp ledge formed by the inferior border of the metencephalic fossa. The pituitary fossa is large, deep, and tubular. The hinder border of the fossa-the dorsum sellce-forms a tumid lip. Anteriorly the fossa is bounded by the pre-pituitary ridge, which forms a broad triangular optic platform. This platform is carried far forward into the interorbital septum, rising gently in its course, and then turning abruptly backwards to terminate at the pre-optic ridge, which may be traced into the tentorial ridge. The form of the optic platform contrasts strongly with that in some other types, e. g. Accipitres. The tentorial ridge in the region of the pre-optic platform is but feebly developed. On the parietal wall, however, it becomes tolerably distinct, especially in Geococcyx and the Musophagi. In these, on its way to the median falx it meets, near the crest of the epiotic, the sharply-defined internal border of the mesencephalic fossa, and forms therewith a prominent angular boss of bone, especially well marked in Geococcyx. The bony falx is not very strongly developed. The oculo-motor (iii.) and abducens (vi.) nerves leave by a common aperture, forming a deep groove across the posterior angles of the optic platform. The cerebral fossce lie quite in front of the cerebellar fossa, and are more flattened dorso-ventrally in the Cuculi than in the Musophagi. The olfactory fossce are relatively feebly developed. The Premaxilla. The external, as distinguished from the palatal, portion of the upper jaw is made up mainly by the premaxilla. This region of the jaw, in the Cuculi and Musophagi, varies considerably in form. Normally, it may be described as about as long as the cranium. In Crotophaga it is slightly longer than this, in Geococcyx and Saurothera very much so. Typically, it may be described as hemicrescentic in outline, depressed from above downwards, and tapering from the base forwards. In Geococcyx and Saurothera it is long and pointed; in Crotophaga surmounted by a sharp, high keel; in the Musophagi more or less inflated. The premaxillary portion of the jaw is slightly decurved at the tip, and the palatal surface is level with the culmen. The nasal processes of the premaxilla fuse completely with the nasals. A nasal hinge occurs in Turacus among the Musophagi, but is wanting in the Cuculi. The Maxillo-jugal Arch. The maxilla is indistinguishably fused with the premaxilla. The maxillo-palatine processes are expanded horizontally, and meeting the ventral border of the nasal septum in the middle line, form an indirectly desmognatlious palate. |