PHTLOGENY OF THE PAL^OGNATH^ AND ^'EOGNATH.t:. 201 



nasals and the free end of the inner spur of the hichryniul. lu Dromwus the relations 

 are similar, but the uasal spur is shorter. 



In Casuariufi the forward extension of the frontal is curtailed, since it terminates 

 Avith the mesethmoidal region of the orbital plate, at the level of tlie origiu of the 

 antorbital plate. The greater part of its mesial border is much thickened and inflated ; 

 this region being involved in the formation of the casque, and reiiresenting its hindmost 

 postero-lateral boundary. In some species this inflation of the bone extends backwards 

 as far as the parietals. 



Fiu'thermore, it is significant to note the fact that the i'rontals fail to meet in the 

 middle line anteiiorly, where they under-floor the median ossification of the casque 

 (PL XLIV. fig. 4). Thus a portion of the ventral border of the casque comes to take 

 part in the formation of the roof of the cerebral fossa. It would seem, from this, as 

 though this secondary ossification was gradually engrafting itself more and more 

 completely upon the skull, by absorjition of the underlying bone in much the same way 

 as bone has replaced cartilage in other cases in the cranial skeleton. 



The extreme anterior end of the mesial border of the frontal is gently hollowed out 

 so as to form the external lateral boundary of the superficial lozenge-shaped plate of 

 the mesethmoid. 



In Ajjterya: the frontal presents one or two points of difi'erence from that of the 

 remaining forms associated herewith. 



Its mesial border joinis its fellow of the opposite side, and its hinder border is 

 bounded by the parietal, as usual. Its postero-veutral or lateral border joins the 

 aliethmoid. Immediately in front of this, the frontal sends downwards and inwards a 

 narrow plate of bone which, eventually, meets its fellow of the opposite side in the 

 middle line, immediately behind the crista galli. From the outside, this orbital plate 

 of the frontal gives oft' a short slender plate to the aliethmoid. 



Cephalad of this orbital plate is a dee[) notch, the supra-orbital fontanelle. This 

 fontanelle is bounded anteriorly by a narrow bar of bone, called the descending proces.'i 

 of tlie frontal, which at its ventral extremity joins the aliethmoid. Mesiad of this 

 bar — between its upper and the outer wall of the aliethmoid — the orbito-nasal nerve 

 passes from the orbit to the nasal cavity. 



The squamosal, in nil the forms herein dealt with, entirely conceals the pro- and 

 opisthotics. In the embryo IJroDUi'iis, which forms the subject of this paper, it is almost 

 quadrangular, its antero-ventral angle is produced into a blunt process which extends 

 downwards to the level of the quadrato-jugal articulation. This downward process of 

 the squamosal is thickened on its inner surface and so closely applied to the quadrate 

 as to render it immovable. 



In the embryo skulls of Dromceus, the squamosal and the parietal are connected 

 by means of a close suture, the juxtaposition of the two elements forming a perfectly 

 level surface. Posteriorly the squamosal rests U2)on the outer border of the exoccipital. 

 and later fuses therewith. Similarly the squamosal, anteriorly, rests U2)on the ali- 



