696 COMPARATIVE ANATOMY 



Epiotic and a sphenotic, in the teleosts and a few other forms de- 

 veloped from the lateral wall of the otic capsule and lying- in front and 



Pterotic, lying behind and directly above the horizontal semicircular 

 canal of the ear. 



The otic bones usually fuse and form a petrosal bone in all the 

 higher forms. This lies directly between the lateral parts of the basi- 

 occipital and the sphenoid. 



A ring of sclerotic bones is often formed from the sclera of the eye 

 of birds and reptiles, though these never unite with the regular bones. 

 From the nasal capsules a lateral ethmoid often forms on the upper wall, 

 while the turbinate bones form on the medial and lateral walls. In man 

 the single occipital bone is formed by the four occipitals mentioned 

 above. The single sphenoid is a fusion of the six sphenoids mentioned 

 above, the alisphenoids form the greater wings and the orbito sphenoids 

 the lesser ones. (Fig. 408.) The ethmoid is similarly made up of the 

 various ethmoids mentioned. 



The mernbranocranium gives rise to the following bones : 



Nasal, covering the olfactory region. 



Frontals, between the orbits. 



Parietals, on the same level with the otic capsules. 



Inter-parietal, unpaired, between parietal and supraocciptal. 



While these are practically all of the membranocranial bones in the 

 roof of the cranium of the higher forms, others may appear in the lower 

 groups. For example : 



Supratemporal, lying lateral to each parietal. 



Postfrontal, behind the orbit. 



Postorbital, forming the posterior wall of the orbit. 



Supraorfcital, taking the place of the frontal in forming the superior 

 or medial wall of the orbit. 



Prefrontal, bounding the orbit in front. 



Lacrimal, lying lateral to the prefrontal. 



Intel-temporal, lying dorsal or medial to the alisphenoid. 



Postparietal, between parietals and interparietals. 



Epiotic, lying above each otic capsule and usually called the tabu- 

 lare. 



If as in some of the fish, birds, and reptiles, the basilar plate and 

 trabeculae fail to ossify, then the roof of the mouth, which is also, of 

 course, the frontal of the cranium, is also a membrane bone called the 

 parasphenoid, while farther forward the vomers or plough-share bones 

 are also membranous and lie in the nasal region. Both parasphenoid 

 and vomers may bear teeth. 



As soon as bone commences to form, the pterygoquadrate changes 

 considerably, becoming closely connected with the cranium in front. 

 The middle portion disappears and the palatines, a pair of membrane 

 bones, replace the disappearing part. The remaining portion of the 



