OCR Text |
Show 1899.] OSTEOLOGY OF T H E PYGOPODES. 1031 cavity, being covered by the prootic. It appears, however, as a small semicircular tract of bone curving round the outer lateral border of the prootic, and bounded above by the frontal and in front by the alisphenoid. The nasal extends backwards to a point corresponding in the adult with the level of the free posterior border of the anterior portion of the fenestrate interorbital septum. The external processes extend forwards to the anterior extremity of the external nasal fossa. The form of the nasal cleft is holorhinal. The posterior border of the nasal is produced backwards into a point. In the Grebe this point is separated from its fellow* of the opposite side by a median forward extension of the frontal; in the Diver the two points meet in the middle line. The mesial edges of the right and left sides are separated one from another, for a considerable distance behind the level of the posterior narial aperture, by the nasal processes of the premaxilla. In the adult the extreme posterior limit of these processes is indicated by a more or less well-marked transverse groove, corresponding with the " nasal hinge " in forms in which this is present. The lachrymal in the Grebes is subcrescentic in form, and apparently in process of degeneration. In the Divers it is still a moderately large bone. It articulates entirely with the nasal, from which it projects laterally, for a considerable distance on either side of the skull, in the form of a subcrescentic backwardly directed spur. The inner border of this spur is hollowed and forms the anterior limit of the supraorbital groove. The extreme posterior end of the spur anchyloses with the supraorbital ledge, which anteriorly between itself and the inner border of the lachrymal is deeply hollowed; thus a large supraorbital fenestra is left for the passage of the duct of the nasal gland. A similar supraorbital fenestra is found also in many Charadriiform birds, e. g. Alcidae. The inferior limb of the lachrymal in the Divers extends downwards to within a short distance of the quadrato-jugal bar. Its free end is deeply notched, the lower projecting backwards for a considerable distance beyond the level of the upper limb of the notch. The premaxilla in the Pygopodes is produced forwards into a point. The median cleft dividing the nasal processes does not extend so far forwards as in the Impennes. They rest upon the internal processes of the nasals. The maxilla extends backwards rodwise to form the inferior border of the anterior half of the quadrato-jugal bar, underlying the jugal, and forwards as a long triangular splint below the maxillary process of the premaxilla. It is bounded on its inner side by the palatine. The maxillo-palatine process takes the form of a concavo-convex lamella, which in the Colymbi extends further backwards into the lachrymo-nasal fossa than in the Podicipides. There is never more than a vestige of the antrum of Highmore. The jugal takes the usual elongated splint-like form. It overlies the maxilla anteriorly and the quadrato-jugal posteriorly. |