SKELETON 77 



iloor 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 addi- 

 tional pair of membrane bones, the vomers. Both vomers and para- 

 sphenoids frequently bear teeth and their origin by fusion of the 

 bases of teeth is clearly seen in developing amphibia (fig. 73). 



Some think the parasphenoid the homologue of the naammalian 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 pterygoquad- 

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

 closely connected with the cranium in front, its median portion dis- 

 appearing, even as cartilage, and being replaced by a pair of mem- 

 brane bones, the palatines (fig. 74), which He between the pre- or 

 parasphenoid and the vomers. The rest of the arch ossifies as two 

 bones on either side, an anterior pterygoid 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 

 pterygoquadrate to form the functional upper jaw (figs. 74, 75) 

 in all bony vertebrates. In its fullest development it consists of 

 bones on either side, beginning behind with a squamosal, which 

 overlies the quadrate, and followed by a quadratojugal, a zygomatic 

 (malar or jugal), and a maxillary, which joins the premaxillary, the 

 latter forming the tip of the jaw and meeting its fellow of the opposite 

 side. Of these only the maxillary and premaxillary bear teeth. 



In the lower vertebrates the roof of the skull is continuous, its 

 only openings being those for the nares and the orbits. In the higher 

 groups vacuities or fossae appear in the postero-lateral parts, these 

 being bounded by bars or arcades of bone. At most there may be 

 three of these fossae. The more lateral of these, the infratemporal 

 fossa (fig. 76), is bounded laterally by the zygomatic and quadrato- 

 jugal, while on the inner side it is separated from the supratem- 

 poral fossa by a squamoso-postorbital arcade. The posttemporal 

 fossa lies between parietal, supratemporal and occipital bones. 

 '< casionally only the infratemporal fossa is present, or, by disap- 

 pearance of the intervening arcade, infra- and supratemporal fossae 

 may unite in a single temporal fossa. Lastly, by the breaking down 



