OCR Text |
Show 1024 MR. W. P. PYCRAFT ON THE [Dec. 19, reduced to a spine-like process dividing two greatly reduced passages for the olfactory nerves, the crura lying caudad of the crista, and not passing on either side. The aliethmoids are the only ectoethmoidal ossifications in either the Impennes, Tubinares, or Pygopodes. They constitute the antorbital plates. In the Colymbi, when present, they resemble in form those of the Tubinares-plates of bone jutting out from the mesethmoid to the lachrymal, sloping obliquely forwards and downwards. They appear, however, in the Colymbi to be but rarely ossified, and never so well developed as in the Tubinares. In the Podicipides their true nature is well seen. Here, they appear as scroll-like bars of bone running continuously backwards, downwards, and inwards from the expanded dorsolateral plates of the mesethmoid itself, to pass eventually into the middle of its posterior border. The olfactory chamber is of comparatively small size. The lachrymal in the Podicipides is free, small in size, roughly semilunar in shape-with the convex border forwards-and apparently a disappearing structure. It articulates by its superior limb with the outer border of the nasal bone. Though conspicuous from a lateral view, it is scarcely if at all visible from the dorsal aspect of the skull. In the Colymbi, the lachrymal is roughly of the same shape as in the Podicipides ; but it differs therefrom, markedly, in several points. Its superior limb, as in the Grebes, articulates with the nasal; but it is free only in the young bird, later it becomes indistinguishably fused with that bone; moreover, it sends backwards a long spur to fuse with the supra-orbital ledge, and to enclose with its aid a passage for the lachrymal duct, as in many Alcidae. Its lower limb, at its free end, is more or less markedly sigmoidally curved. It is, on the whole, a larger and stronger bone than in the Grebes. The Cranial Cavity.-The metancephalic fossa of the Pygopodes is relatively both longer and shallower than in the Impennes and Tubinares, but is deeper in the Grebes than in the Divers. The vagus foramen occupies relatively the same position as in the two last mentioned groups; the two condyloid foramina, similarly, lie mesio-caudad of this. The internal auditory meatus lies immediately under the mouth of the floccular fossa and in front of the vagus foramen. The abducent foramen pierces the anterior border of the fossa, passing on either side of the pituitary fossa, aud emerging in the Grebes within the rim of the ventral border of the optic foramen, and in the Divers caudad of the foramina for the oculomotor nerve and internal ophthalmic artery. The cerebellar fossa agrees with the Tubinares, and differs from the Impennes, in its greater relative size. It lacks, however, the transverse system of grooves and ridges representing the cerebellar sulci and gyri so marked a feature in this region of the skull in the Tubinares. |