226 ON THE STRUCTURE OF THE SKULL. 



but it involves that of certain other bones of the side walls of the 

 skull. 



In the Clielonia, and in many Lizards, the lateral walls of 

 the cranium, between the pro-otics and the prefrontals, are 

 entirely occupied by cartilage, or by membrane. In the dry 

 skull of the Turtle (Fig. 88, C) it is true that there is. an ap- 

 parent bony wall in front of the pro-otic, but this is only a 

 process sent down from the parietal, which becomes connected 

 with the pterygoid, and with a small, distinct lamella of bone. 



In Lizards a distinct, rod-like bone (Co, Fig. 90, A, and 

 Fig. 91), occupies a corresponding position, articulating above 

 with the parietal, and below with the pterygoid, and receives 

 the name of the columella. 



Fig. 91. 



,1 



Pa 



Mt 



BS^Pt 



Fig. 91. — The inter-orbital septum of a Lizard (Iguana), B S, the anterior prolongation 

 or beak of the basi-sphenoid ; a, the inferior median ossification; 6, the superior paired 

 ossification of the left side of the inter-orbital septum ; Co, the columella. 



In both Chelonians and Lizards the basi-cranial axis is 

 laterally compressed in the presphenoidal region, and is con- 

 verted into a vertical inter-orbital septum, as in the Pike. In 

 the Chelonians, neither the septum, nor the membranous, or 

 cartilaginous, alisphenoidal and orbito-sphenoidal regions con- 

 nected with it, present any ossifications, but, in many Lizards, 

 delicate laminae of bone are developed in this region. In the 

 Iguana tuberculata, for example (Fig. 91), the inter-orbital 

 septum is supported below by the prolonged beak of the basi- 

 sphenoid. Above this, it presents a long median jn-esphenoidal 

 ossification (a) forked posteriorly. The forks are connected 

 with two bones, one on each side (b), which appear to represent 

 orbito-sphenoids. 



