OCR Text |
Show 392 MB. W. P. PYCRAFT ON THE [Mar. 21, In Prion there is a tendency towards a distal expansion of the pterygoid, so marked a feature in the Sphenisci. Ill-defined basi-pterygoidal facets can be traced on the inner border just behind the point where the shaft rests upon the parasphenoidal rostrum ; just within the inner border of the quadrate articular end is a small pneumatic foramen. In the Diomedeidas the pterygoids are relatively longer than in the Procellariidas, they bear no trace of basipterygoidal facets, are quite rod-shaped, and rest upon the parasphenoidal rostrum only by the inner border of their extreme distal ends. The posterior pneumatic foramen is very large. In Thalassogeron the extreme distal end rises, upwards, above the pterygoidal articulation to embrace the rostrum. In Diomedea exulans only does there seem to be a total absence of a dorsal crest. The quadrate differs from that of the Storks mainly in the disposition of the mandibular articular surfaces, in the absence of a pneumatic foramen between the posterior surfaces of the otic and squamosal articular surfaces, and the less marked division between the dorsal aspects of these two processes. The dorsal border is slightly hollowed; the orbital process large and expanded. The mandibular articular surface is very broad, runs at right angles to the long axis of the skull, and projects inwards considerably beyond the base of the orbital process. There is a very distinct head for articulation with the pterygoid. The outer mandibular condyle is marked by a strong median transverse depression, slopes obliquely backwards, and is separated by a wide groove from the inner, which takes the form of two grooves divided by a median ridge. The quadrato-jugal glenoid cavity lies in the outstanding process at the base of the outer side of the otic process. The Mandible. As in the Penguins, Storks, and Herons, tbe dentary suture and the spatulate free end of the coronoid (fig. 1) remain distinct through- Fig. L Inner (A) and outer (B) views of the lower jaw of a nestling Oceanodroma leucorrhoa. any., angulare; ar., articulare; d., dentary ; cor., coronoid ; s.a., supra-angular sp., splenial. |