OCR Text |
Show 382 MR. W. P. PYCRAFT ON THE [Mar. 21, Tubinares in the very valuable memoirs of Milne-EdAvards (14), Brandt (3), Huxley (12), Forbes (5-6), Gadow (8-9), Lydekker (13), and others. Nevertheless, in working carefully through the collection of skeletons of this group in the British Museum, I found that much yet remained to be done, in the way of bringing these facts together, so that, carefully sorted, they might be brought yet more fully to bear upon the question of the systematic position of the group. In this I think I have had a fair measure of success. Besides also I have been enabled to add, here and there, a few original observations. Following the plan of m y last paper, I propose first of all to deal with (ii.) the Adult Skull, then with that of (iii.) the Nestling, folloAving this with (iv., v.) the Axial Skeleton, (vi.) the Sternum and Pectoral Girdle, (vii.) the Pelvic Girdle, (viii.) the Pectoral Limb, and (ix.) the Pelvic Limb. ii. THE SKULL OF THE ADULT. The skull of the Petrels, like that of the Impennes and Colymbi, is schizognathous, holorhinal, and marked by deep supraorbital grooves ; but it can at once be distinguished therefrom by its large, laterally expanded vomer fused posteriorly with the palatines, an olfactory cavity of great size-except in Pelecanoides and Pufjlnus assimilis,-and the markedly hooked upper jaw. The mandible retains a distinct dentary suture and coronoid, the free end of which last terminates in a more or less heart-shaped expansion. The angulare is truncated, and the internal angular process is small. The Occipital Region.-The dorsal border of the supra-occipital region in the Procellariidas is strongly arched ; in the Diomedeidas the curve of this border is very slight. The curve is produced downwards on either side into the paroccipital processes. Avhich project, or rather depend, from the skull in the form of conical " bosses." The aperture of the foramen magnum varies in form and size. The occipital condyle is sessile, save in Diomedea exulans, in which it is produced backwards on a stout base so as to project considerably behind the foramen. In certain genera- e. g., Thalassceca, Daption, CEstrelata, Prion, Priocella, some species Pufjinus, Oceanites, Cymodroma, Pelagodroma, and Procellaria-the supra-occipital presents the concavo-convTex form so characteristic of the Sphenisci. In other forms this swelling is hardly perceptible. The Roof of the Cranium.-The fronto-parietal region is more or less furrowed in the median line, thus indicating the position of the pallial cerebral fissure; similarly, in many cases-Thalassceca, Priofinus, CEstrelata, Puffinus-the cerebellar prominence is transversely ridged, the ridges corresponding to the underlying sulci of the cerebellum. The temporal fossae vary much in the extent of their development. In Priofinus, Fulmarus, Majaqueus, Ossifraga, and many species of CEstrelata and Puffinus they rise dorsally so as to be divided only by a narrow median |