SKULL— OSSIFICATION 



73 



parietal foramen for the parietal organ between them (fig. 76) . These 

 are followed by a pair of frontal bones, usually lying between the 

 orbits; these in turn being joined in front by the nasal bones which 

 roof the nasal region above the ethmoid and bound the nares behind. 

 With the appearance of bones in the skull, the function of the 

 pterygoquadrate as the upper jaw is taken by dermal bones developed 

 in the upper border of the mouth, these bones at the same time form- 

 ing the anterior and lateral boundaries of the cranium. At its 

 fullest development this series of bones consists, on either side, of a 

 premaxilla (inter maxilla, incisivum), in front, this forming the tip 



Fig. 75. — Diagram of dorsal side of skull; chondrocranium dotted; cartilage bones with 

 lines and dots; membrane bones outlined. 



of the jaw and bounding the naris in front and below. Lateral to 

 the premaxilla is a maxilla, maxilla and premaxilla usually being the 

 only tooth-bearing bones in the margin of the jaw, while the maxilla 

 usully forms a part of the lateral (inferior) border of the orbit. The 

 maxilla connects behind with the zygomatic (malar, jugal) bone 

 which also forms part of the lateral border of the orbit and is con- 

 nected posteriorly with a less constant element, the quadratojugal, 

 which extends back to. the quadrate and also articulates with the 

 squamosal bone which, in the more primitive Vertebrates forms the 

 postero-lateraj angle of the cranium. The squamosal overlies the 

 quadrate and is often immovably connected with it. 



