SKELETON. 69 



with the supraoccipital) these are practically all of the membrane 

 bones in the cranial roof of the adult. In the lower groups there are 

 several other bones, some of which may appear in the development of 

 the higher forms. Thus lateral to each parietal there may be a su- 

 pratemporal; behind the orbit a postfrontal may articulate with the 

 frontal, and lateral to this, and forming the rest of the posterior wall of 

 the orbit a postorbital. Occasionally the superior (or medial) wall of 

 the orbit is formed by one or more supraorbital bones, which, when 

 present, exclude the frontal from the orbit. The orbit may be bounded 

 in front by a prefrontal bone, adjoining the antero-lateral margin of 

 the frontal, and lateral to this there is usually a lacrimal bone. Less 

 constant are an intertemporal bone dorsal (medial) to the alisphenoid, 

 a pair of postparietal bones between parietals and interparietals and 

 a so-called 'epiotic' above each otic capsule, which, since it is not a 

 cartilage bone and has no relation to the true epiotic, is better called 

 the tabulate. 



In the ichthyopsida, and to a less extent in the sauropsida the 

 basilar plate and trabeculae may fail to ossify. In these cases the floor 

 of the cranium (roof of the mouth) is formed by a membrane bone, 

 the parasphenoid, which lies ventral to the cartilage in the sphenoid 

 region. Farther forward, in the nasal region, are an additional pair 

 of membrane bones, the vomers. Both vomers and parasphenoids 

 frequently bear teeth and their origin by fusion of the bases of teeth 

 is clearly seen in developing amphibia (fig. 65). 



Some think the parasphenoid the homologue of the mammalian vomer, calling 

 the vomers of the non-mammals prevomers, their representatives being sought in 

 the 'dumb-bell bone' of the monotremes. More evidence is needed on these 

 points. 



With the appearance of bone the mandibular arch undergoes the 

 greatest modifications of all the visceral arches. Its pterygoquadrate 

 half loses its function as the upper jaw and becomes more closely 

 connected with the cranium in front, its median portion disappearing, 

 even as cartilage, and being replaced by a pair of membrane bones, 

 the palatines (fig. 66) , which lie between the pre- or parasphenoid and 

 the vomers. The rest of the arch ossifies as two bones on either side, 

 an anterior pterygmd and a posterior quadrate, which now becomes 

 the suspensor of the lower jaw. In the teleosts and reptiles there are 

 a series of pterygoid bones. 



A second arch of membrane bones develops outside of the pterygo- 



