1899.] OSTEOLOGY OF THE PYGOPODE8. 1031 



cavity, being covered by the prootlc. It appears, however, as a 

 small semicircular tract of bone curving rouud the outer lateral 

 border of the prootic, and bounded above by the frontal and in 

 front by the alispbenoid. 



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 holorhiual. 

 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 subcresceutic 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 Charadriif orm birds, e. y. Alcidse. 

 The inferior limb of the lachrymal in the Divers extends down- 

 wards 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 hmb of the 

 notch. 



The premaodlla 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 over- 

 lies the maxilla anteriorly and the quadrato-jugal posteriorly. 



