OPHTHALMOSArEUS. 19 



region. The form of the articulation (text-fig. 9, C) varies somewhat, according to the 

 degree to which the ossification of the cartilage has proceeded ; in the most fully 

 ossified specimens the surface is strongly convex from before back. In the transverse 

 direction there is a double curve, the inner portion being convex, especially towards 

 the middle of its length, Avhile the outer is concave, especially in front. The prominent 

 postero-external angle of this region forms a well-marked projection {q^j-f-), with the 

 upper end of which the quadrato-jugal articulates by a concave surface {if-i 

 text-fig. 10, A-C). A little above the articular end the bone narrows to form a 

 sort of wide neck (n.) : the anterior border of this region, when fully ossified, is sharp 

 and gently concave ; the posterior border is thickened and rounded and forms the 

 lower part of the deeply concave posterior border, the degree of concavity varying 

 greatly in different individuals. The upper and anterior border of the blade {a.h.) 

 forms a long convex curve forwards to the upper end of the neck, with which it makes 

 an obtuse angle {a.i.a.) : the edge is thickened along the upper end and thinner 

 anteriorly ; throughout its length it seems to have been bordered with cartilage. 

 The outer face of the blade is concave in all directions, and its posterior upper angle 

 {l).e.a.) is bent sharply outwards ; the inner face is divided into a postero-superior 

 portion bent outwards, and a nearly flat anterior region which is overlapped by the 

 squamosal above and the pterygoid below. The pterygoid is intimately united to 

 the roughened surface marked in the figure by a dotted line {pt.) ; it termiuates above 

 by a thin tongue-like expansion, which in its turn is overlapped by the squamosal {sq.). 

 Immediately behind the pterygoid plate, and in fact forming a notch on its posterior 

 border, is the facet (s^._/".) with which the outer end of the stapes unites. The squamosal, 

 as already mentioned, embraces the upper end of the bone and on the inner f;xce 

 overlaps it and part of the pterygoid. The whole arrangement of the quadrate 

 (see text-fig. 4) seems to be directed towards the attainment of the greatest possible 

 rigidity: thus, on the inner side it is supported by the pterygoid, which, in its turn, 

 is closely united with the basisphenoid ; by the squamosal, which is joined to the 

 parietal, supratemporal, and postfrontal, and is further braced by the outer end of 

 the opisthotic and by its overlap on to the pterygoid ; by the stapes, which extends 

 from the above-mentioned notch to the basioccipital, and is further held in position by 

 the inferior plate of the pterygoid, on which it rests. On the outer side also the 

 quadrate is supported by the quadrato-jugal, though in this case the mode of union 

 seems to have allowed a little movement between the two bones. The arrangement in 

 the Khynchocephalia is, of course, similar in many respects, but neither in that nor 

 in any other group of reptiles does it appear that the stapes has been converted into 

 a mere support for the quadrate. 



The quadrato-jugal (text-fig. 10, A-C) is a small triangular bone, the postero-inferior 

 angle being obtuse : both the posterior and lower borders are thickened, while the 

 anterior (upper) edge is thin and sharp. The posterior loAver angle is occupied by a 



d2 



