974 MR. W. p. PYOBAFT ON THE [DeC. 13, 



the former, the pterygoid takes the form of an obliquely placed, 

 flattened lamina tapering to a point forwards. The ventral border 

 of its anterior half rests upon the flange-like projection from 

 the dorsal border of the posterior limb of the paired vomer : the 

 two becoming, in the adult, entirely fused so as to leave no trace of 

 the line of their union. 



In Rhea it is of exactly the same shape anterioi'ly as the small 

 splint in Splieniscidaj, and runs forward along the dorsal border of 

 the posterior eud of the vomer, just as, only to a greater extent, it 

 does in the young Carinate skull. In the case of Rhea, however, 

 the inferior surface of this eud of the pterygoid is grooved, and 

 into this groove the postero-internal angle of the palatine and the 

 outer superior border of the posterior limb of the vomer are received. 

 That this " bemipterygoid " ', as I propose to call this anterior 

 segment of the pterygoid, in the Carinatse, is a part of the true 

 pterygoid, and that it represents an earlier phase when the relations 

 between the pterygoid and vomer « ere precisely similar to what 

 obtains amongst the Eatitse and Tiuaiuous at the present day, is 

 highly probable. Originally then, in the " Carinatse," as in the 

 " Eatitae " now, the pterj'goid terminated in a point restiug on 

 the vomer and was unsegmented ; since, it has divided into an 

 anterior and a posterior moiety, a joint forming between the seg- 

 ments. The pterygoid of the adult represents only the posterior 

 and larger portion, the anterior having fused with the palatine. 

 This state of things may probably be interpreted as the result of 

 mechanical stress causing a fracture at the weakest part of the bone, 

 such stress being brought about by the shifting of the palatines 

 towards the middle line from their originally dromseognathous 

 position. 



Prom the relations between pterygoid and vomer, we may turn 

 profitably to the relations between pterygoid and palatine. 



In the Eatitae, as repi-esented by Dromans and Rhea, the palatines 

 are more or less triangular in form and do not extend forwards 

 beyond the level of the posterior border of the maxillo-palatine 

 processes. In the Carinatse they extend forwards by means of 

 a rod-like spbnt nearly to the tip of the upper jaw. 



In Dromanis the palatine is attached to the outer border of the 

 dorsal surface of the posterior limb of the vomer on each side, and 

 is not in any way connected with the pterygoid. Thus, on a 

 ventral view of the skull, the vomer is continued directly back- 

 wards to the pterygoids, its two posterior hmbs forming with each 

 of the latter a continuous bar. In Rhea the anterior half of the 

 internal, or mesial, border of the palatine articulates with the 

 posterior limb of the vomer of that side, the posterior half with 

 the pterygoid. Thus the vomer appears to be entirely separated 

 from the pterygoid by the palatine. Moreover the latter is being 



1 For this word I have to thank Prof. 6. B. Howes, F.E.S., of the Eo3al 

 College of Science, whose aid I sough t after having failed to coin a name to my 

 own satisfaction. 



