554 



ON THE THEORY OF THE VERTEBRATE SKULL 



these determinations are correct in the bird, their extension to the 

 •other Ovipara is a logical necessity. But the determination of these 

 bones throughout the vertebrate series is the keystone of every theory 

 of the skull — it is the point upon which all further reasoning must 

 turn ; and therefore it is to them, in considering the skulls of the 

 other Ovipara, that I shall more particularly confine myself 



Composition of the Skull of the Tiirtle. 



It has been seen that in birds the presphenoid, ethmoid, and 

 orbitosphenoid regions are subject to singular irregularities in the 

 mode and extent of their ossification. In the turtle, not only are the 

 parts of the cranium which correspond with these bones unossified, 



Fig. 5. — Longitudinal section of the Skull ot a Turtle [Chelone mydas), exhibiting the 

 relations of the brain to the cranial walls. The dotted parts marked AS. OS. PS. and 

 Eth. are cartilaginous. 



but its walls remain cartilaginous for a still greater extent. In fact, 

 if a vertical section be made through the longitudinal axis of a 

 turtle's skull, it will be observed that a comparatively small extent of 

 the cranial wall, visible from within, is formed by bone, and that the 

 large anterior moiety is entirely cartilaginous and unossified. The 

 anterior part of the posterior, bony, moiety of the cranial wall is 

 formed by a bone (Pt.), whose long, vertical, anterior-inferior margin 

 forms the posterior boundary of the foramen by which the third 

 division of the trigeminal nerve makes its exit from the skull. The 

 anterior and superior margin of the bone is very short, and articulates 

 with the parietal bone. The superior margin is inclined backwards, 

 and articulates with the supiaoccipital. The posterior margin is 

 straight, and abuts against a cartilaginous plate interposed between 



