SKULL OSSIFICATION 



77 



Fig. 8o. — Palatal surface of 

 Acanthostoma (Stegocephal, Jaekel, 

 '06). tnx, maxilla; o, orbit; pa, 

 palatine; pmx, premaxilla; ps, 

 parasphenoid; pt, pterygoid; q, 

 quadrate; qj, quadratojugal; t, 

 transversuni; v, vomer. 



Another series of membrane bones, arising primarily by the fusion 

 of the bases of teeth, occurs in the roof of the mouth (cranial floor) ; 

 maxilla and premaxilla belonging to this group. In many lower 

 Gnathostomes the middle part of the oral roof is formed by a para- 

 sphenoid bone (fig. 80) which Hes just 

 ventral to basi- and presphenoid car- 

 tilages, its extent and development being 

 inversely reciprocal with that of the 

 cartilage sphenoid bones. In the higher 

 classes it is reduced or absent.^ When 

 true jaws are developed, the pterygo- 

 quadrate bars no longer meet in the 

 middle line, but may join the sides of 

 the cranium or end freely. Usually in 

 Tetrapoda a large part of the pterygoid 

 cartilage does not ossify, but is overlaid 

 with a membrane bone which is called 

 the pterygoid. (This whole matter of 

 pterygoid homology needs a thorough 

 review.) The pterygoid bars and their 

 associated membrane bones are continued medially and ventral 

 to the cranium by a palatine bone on either side, the palatines 

 of the two sides often meeting in the middle line. Between 

 these (dermo) palatines and the premaxillse is a somewhat similar 

 pair of vomers (unpaired in Teleosts) lying in front of and between 

 the primitive choan^. Parasphenoid, palatines and vomers fre- 

 quently bear teeth, an indication of their origin (p. 7). 



The membrane bones of the visceral arches include those of the 

 oral roof, the margins of the upper jaw and nearly all of the bones 

 of the lower jaw, but dermal bones are practically absent from hyoid 

 and branchial arches. Those of the lower jaw are arranged around 

 Meckel's cartilage which usually disappears, but may persist inside 

 them through life. The extreme of separate bones in the mandible 

 includes the following on either side of the jaw: A dentale (dentary) 

 in front, w^hich, near the tip of the jaw, surrounds the Meckelian and 

 extends farther back on its outer than on its inner side. This is 



1 It has been suggested recently that the parasphenoid is the homologue of the mam- 

 mahan vomer, necessitating a new name (prevomer) for the so-called vomers of the 

 non-mammalian Vertebrates. So far there is little developmental support for this view, 

 and until better evidence appears it is better to retain the older view and names. 



