OCR Text |
Show 1032 MR. W. P. PYCRAFT ON THE [Dec. 19, The quadrato-jugal extends forwards beyond the middle of the quadrato-jugal bar, passing to the inner side both of the jugal and maxilla. It articulates posteriorly with the quadrate, fitting into a deep cup-shaped cavity. The vomer in the Colymbi articulates with the hemipterygoid posteriorly, and with the palatine by means of its dorsal border. In the Podicipides, in our Museum skeleton, the relations of the vomer to the hemipterygoid are not easily made out, owing to the fact that the hemipterygoid has not yet split off from the pterygoid (p. 1026). The palatine is of great length, extending forwards as a long slender rod to within a short distance oP the tip of the jaw. Posteriorly it is more orless grooved along its ventral aspect; from its inner dorsal border there arises caudad a leaf-like plate of bone turning inwards towards the middle line, the free border of which articulates with the superior border of the posterior end of the vomer. The pterygoid, in so far as its general form is concerned, has been already described. W e are concerned here only with the segmentation of its anterior end to form the Hemipterygoid.-This can best be studied in the Colymbi. Here it bears a very close resemblance to that of the Impennes. In the skulls of two Divers in the Museum Collection the segmentation between the pterygoid and hemipterygoid is not only complete, but a perfect joint has formed between the two. The hemipterygoid itself has not yet fused with the palatine, but articulates with it by suture. Its form is that of a short triradiate spike extending forwards above the proximal end of the palatine, which underfloors it, to overlap the extreme posterior end of the vomer, which, as previously remarked, articulates for the most part with the palatine. In the Grebe, in the youngest skulls, segmentation has not yet taken place ; but at the point where this is about to happen there is an indication of a fracture, having jagged edges similar to that figured and described recently in the Impennes, only that in this case the separation is less distinct. In a nearly adult Grebe the form and relations of the hemipterygoid agree exactly with those of the Diver just described. In the Tubinares, it will be remembered, the form of the hemipterygoid differed from that just described. The dentary does not appear to undergo any appreciable change of form between nestling and adult periods. The splenial is precisely similar in form, both in Grebes and Divers. It resembles a flattened cone, the base contributing to form the ventral border of the jaw. The coronoid is at first rod-shaped, then turns abruptly upwards and expands into a flattened trowel-shaped blade, which remains more or less distinct throughout life. The angulare is only just distinguishable as an independent bone. The supra-angulare can be distinguished as a separate bone only in the youngest of the Grebes in the M u s e u m Collection. |