OCR Text |
Show 280 MR. W. P. PYCRAFT ON THE • [Apr. 15, antorbital plate (prefrontal). The horizontal process is practically suppressed. The fronto-parietal region in Gypohierax, Aquila, Falco, Polyborus, and Milvus, the parietal only in Polyboroides, Buteo, and Haliaetus, is marked by a shallow median groove. This groove is more or less traceable in all the Accipitres save the Cathartse and Serpentarius. It is especially noticeable in the forms just enumerated. In the Cathartse the roof of the skull presents an evenly rounded surface. The width across the skull at the fronto-parietal region in no case approaches that of some Owls, e. g. Bubo, owing to the smaller size of the postorbital processes. The fusion of the nasals with the frontals is complete, and leaves no trace of the line of junction. The Base of the Skull. The basitemporal plate in Serpentarius only is visibly thickened by pneumatic tissue. In the remaining members of this group it is a thin triangular plate with a slightly concave ventral surface. Posteriorly it is bounded, in the middle line, by a more or less well-marked precondylar fossa. It extends outwards on either side as a wing-like plate to join the inferior wing of the exoccipital process, in Serpentarius, Eagles, Buzzards, Falcons, and Vultures, for instance. But in the Osprey this junction with the exoccipital-completing the mouth of the tympanic cavity below-is formed only by a thin bar of bone. In certain Vultures and in the Cathartse the hinder angles of this plate appear to terminate in a pair of prominent mammillary processes. They are the dominant features of this region of the plate, and by their great size have come to lie behind the actual posterior angles. The two sides of this triangular plate may have sharply defined free edges, e. g. in Aquila, in which case the Eustachian channels are open grooves; or they may be partly fused with ossified connective tissue forming the anterior wall of the recessus tympanicus anterior, when the grooves are partly closed, e. g. Serpentarius, Haliaetus, Buteo; or they may fuse throughout with the inferior border of the wall forming the above-mentioned recess, leaving only a small Eustachian aperture at the apex of the triangle, as in the Cathartse and Polyboroides, for instance. In the Cathartse the parasphenoidal rostrum immediately above this aperture is deeply excavated. This is especially marked in Sarcorhamphus. The parasphenoidal rostrum may or may not bear basipterygoid processes. These are largest in Serpentarius, where they lie at the base of the rostrum. In the Cathartse they may be either short and broad, as in Catharistes and Gypagus, or very slender and seated further forward on the rostrum, as in Pseudogryphus and Sarcorhcemphus (Condor). This greater slenderness evidently marks the first stages in their decay. In a skull of Sarcorhamphus |