THE SKULLS OF REPTJLIA AND AVES. 241 



Altogether anteriorly, however, where the two trabeculae 

 have coalesced, there grows out of this part, from the two cornua 

 in which it ends, a pair of very delicate cartilaginous plates. 

 At the end of the third period both plates acquire a not incon- 

 siderable size, take the form of two irregularly-formed triangles, 

 and are moderately convex above, concave below, so as to be, 

 on the whole, shell-shaped. The nasal bones are developed 

 upon these, while below them are the nasal cavities, and the 

 nasal glands with their bony capsules. 



The alaa, or lateral parts, of the two sphenoids do not grow, 

 like the lateral parts of the occipital bone, out of the basis cranii, 

 the foundation of which is formed by the cephalic part of the 

 chorda, but are formed separately from it, although close to it, 

 in the, until then, membranous part of the walls of the cranium. 



The alae of the presphenoid (orbito-sphenoids), which are 

 observable not very long before the termination of the third 

 period, appear as two truly cartilaginous (though they never 

 redden), irregular, oblong, plates of moderate thickness ; lie in 

 front of the optic foramina, at the sides of the lateral trabecule 

 of the skull; "ascend from them upwards and outwards, and are 

 somewhat convex on the side turned to the brain, somewhat 

 concave on the other. The alae magnse (alisphenoids) are 

 perceptible a little earlier than these. They are formed 

 between the eye and the ear, and also originally consist of a 

 colourless cartilaginous substance : they appear, at the end of 

 the third period, as irregular four-sided plates ; lie at both sides 

 of the anterior half of the investing plate of the chorda ; ascend 

 less abruptly than the alae orbitales, and are externally convex, 

 internallv concave. 



The upper posterior angle of each elongates, very early, 

 into a process, which grows for a certain distance backwards, 

 along the upper edge of the auditory capsule, and applies itself 

 closely thereto. 



The auditory capsules, or the future petrous bones, chondrify, 

 as it would appear, the earliest of all parts of the skull : the 

 fenestra ovalis arises in them by resorption. 



The ossification of the Snake's skull commences in the 

 basi-occipital, or, at any rate, this is one of the first parts to 



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