OCR Text |
Show 1899.] OSTEOLOGY OF THE PYGOPODES. 1023 The orbitosphenoid in the Divers is completely ossified; in the Grebes this is largely represented by membrane, so that the anterior wall of the brain-case in the dried skull bears a more or less considerable fenestra. In a skull of JEchmophorus kindly lent m e by M r . Beddard this fenestra is very small, the orbitosphenoid is ossified dorsad so as to close iu and form tubes for the olfactory nerves as they leave the brain. In the Divers the interorbital septum forms a vertical bar in front of the optic foramen, this is wanting in the Grebes. The ethmoidal region.--The mesethmoid is indistinguishably fused with the presphenoid ( = interorbital septum) behind, and the parasphenoidal rostrum below; it expands dorsally as usual into a pair of lateral alietbmoidal plates under the nasal and frontal bones, the free edges of which curve slightly downwards under the outer border of the frontal and along the inner border of the lachrymal. Its postero-dorsal border is continued backwards to terminate in a sharp, spinous, crista-galli forming a median partition between the olfactory nerves. The antorbital plate in the Divers is represented by a thin ridge of bone running from the mesethmoid outward and forward to the lachrymal. In the Grebes this is represented by a narrow band-shaped scroll of bone from the lower and hinder border upwards to the nasal, immediately to the inner side of the dorsal end of the lachrymal. A comparison may profitably be made here between the mesethmoid of the Pygopodes and that of the Impennes aud Tubinares. In the two first mentioned groups the mesethmoid is relatively smaller than in the last, and only very slightly pneumatic. In the Divers and Grebes its anterior border curves gently forwards, carrying with it a pair of lateral wing-like ridges, the whole eventually terminating in a sharply truncated border running transversely across the skull immediately under the free end of the nasal processes of the premaxilla. Its posterior border is deeply hollowed by the interorbital fenestra. Its dorsal border, anteriorly, expands into a pair of lateral aliethmoidal plates, tapering from before backwards; posteriorly it runs backwards, in the Divers in the form of a deep, and in the Grebes in the form of a very narrow knife-like ridge, the free end of wdiich terminates as a pointed " crista-galli" within the olfactory fossa. In the Impennes the form and relations of the mesethmoid closely resemble those of the Pygopodes. In the Tubinares, the mesethmoid differs from the forms de~ scribed on account of the fact that its upper and lower regions are brought into sharp contrast by reason of the great pneumaticity of the lower region, which causes the upper non-pneumatic half, with its gently arched aliethmoidal wings, to assume a cavern-like form, which passes backwards in a tubular manner into the olfactory fossa. Moreover, the crista-galli takes the form of a median' pillar dividing two large tubular apertures for the olfactory crura; whilst in the other forms the crista-galli is 66* |