76 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



In the stegocephals, reptiles and birds the sclera of the eye often 

 gives rise to a ring of sclerotic bones (fig. 75), which, however, never 

 unite with the other bones of the skull. The nasal capsules often 

 develop a lateral ethmoid on the upper wall, and turbinals on the 

 medial and lateral walls. 



To place these bones in the terms of human anatomy: the four occipitalia 

 fuse to form the single occipital of man; the six sphenoidalia similarly unite to 

 form the single sphenoid, the alisphenoids forming the greater wings, the 

 orbitosphenoids the lesser wings; while the ethmoidalia fuse to the ethmoid. 



In all bony vertebrates the cranial walls are completed dorsally 

 by membrane bones, which in the lower fishes overlie the tegmen 

 cranii, while in the higher groups they replace it, the cartilage failing 

 to develop in the roof. The number of these elements varies between 

 wide Hmits, the following being the most constant. 



Beginning in front (fig. 75), there are, on either side of the median 

 line a pair of nasal bones covering the olfactory region; a pair of 

 frontals between the orbits; a pair of parietals at the level of the 

 otic capsules, between which there is frequently a parietal foramen 

 for the connexion of the parietal eye with the brain; and an inter- 

 parietal, arising from paired centres, between the parietals and the 

 supraoccipital. 



In the higher vertebrates (where the interparietal frequently fuses 

 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 tabulare. 



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

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



