186 ME. W. P. PTCEAFT ON THE MORPHOLOGY AND 



From its dorsal surface in Dinornis there arises a large hollow shell of bone, the inner 

 wall of which embraces the triangular plate of the rostrum. The cavity of this bony 

 shell — the antrum — opens posteriorly by a large foramen into the quadrato-jugal fossa. 

 It articulates with the palatine by a short recurrent process from its postero-internal 

 angle. 



In Emeus there is no antrum. AVhat corresponds to its inner wall in Dinornis exists 

 as a flat concavo-convex vertical plate applied to the anterior border of the mesethmoid 

 triangular process and the rostrum. 



In Rhea the maxilla is almost entirely represented by the maxillo-palatine process. 

 Viewed dorsally it is seen to take the form of a flat, triangular, perforate plate of bone, 

 with its sharply pointed end running forwards to within a short distance of the 

 anterior end of the external narial aperture. This more pointed region rests upon the 

 palatine process, and is bounded externally by the maxillary process of the premaxilla. 

 Its postero-mesial border is bounded by the vomer. Its hinder end presents free 

 postero-internal and external borders. The former lies more or less markedly forward 

 than the latter, and bounds the posterior nares anteriorly. The latter lies more or less 

 considerably candad, and bounds the anterior end of the quadrato-jugal fossa. Its 

 postero-internal angle is produced backwards into a long spiue, which is closely 

 applied to the outer border of the palatine, extending backwards for a considerable 

 distance. Its postero-external border is also produced backwards spine-wise, and 

 underlies the quadrato-jugal bar. From the dorsal surface of the maxillo-palatine 

 plate there arises a long slender style which, directed upwards and backwards, ultimately 

 articulates with the anterior angle of the lachrymal, and thus serves to divide the 

 lachrymo-nasal (antorbital) from the nasal fossa (PL XLIII. fig. 9). In all other 

 birds this is done by the descending maxillary process of tlie nasal. The antrum is 

 represented only by a minute aperture at the base of the vertical rod — the lachrymo- 

 nasal process of the maxillo-palatine — ^just described. 



The quadrato-jugal fossa extends forwards nearly as far as the level of the anterior 

 end of the posterior nares. 



In Struthio the maxilla is splint-shaped, produced posteriorly into two spine-like 

 processes, the inner of which runs dorsad over the distal end of the palatine, the outer 

 running ventrally below the quadrato-jugal bar. Anteriorly it extends forwards 

 nearly as far as the body of the premaxilla. The maxillo-palatine processes are small 

 and hatchet-shaped ; their mesial borders clamp the vomer, which is grooved laterally 

 for their reception. There is a small shallow antrum, the roof of which is fenestrated. 

 The anterior border of the maxillo-palatine process bounds the enormous palatine 

 foramen posteriorly. 



The ossified style, described in Rhea as dividing tlie lachrymo-nasal from the nasal 

 fossa, is here represented by a ligament springing from the external dorsal angle of the 

 mouth of the antrum and inserted into the short descending process of the nasal. 



