SKULL 



213 



certain investing dermal bones (Fig. 123, A). A pair of large 

 frontal bones form the cranial roof, and also help to roof in the 

 orbital cavities. Behind the frontals, and separated from each 

 other by the supraoccipital, there is a pair of small parietals, and 

 anterior to the frontals a median dermal mesethmoid. A small 

 nasal bone overlies each olfactory recess. Ventrally, the base of 

 the cranium, from the basi-occipital to the prenasal region, is 

 strengthened by a large parasphenoid behind, and a much smaller 

 vomer in front, both of which lie in the roof of the mouth. The 

 vomer is a tooth-bone, and probably the parasphenoid also. 



The mandibular arch (Pig. 123, B) is more modified than that 

 of the Dog-Fish. The palato-pterygo-quadrate bars, or primitive 

 upper jaw, no longer meet in front beneath the cranial floor, but 

 each separately articulates in front with the lateral ethmoid of its 



Fig. 122. —Vertical and 

 longitudinal section of 

 tlie cranium of Salmo 

 salar, showing the right 

 half of tlie cranial 

 cavity. Cartilage is 

 dotted. /, Frontal ; v', 

 fat-containing cavity in 

 the mesethmoid carti- 

 lage ; V, VII, IX, X, 

 foramina for the fifth, 

 seventh, ninth, and 

 (From W.K.Parker.) 



ule 



tenth cranial nerves. Remaining reference-letters as in Fig. 121 



side. Although still partly cartilaginous each bar is largely replaced 

 either by cartilage bones, or by bones which begin as membrane 

 bones or as tooth-bones and complete their growth by invading 

 the cartilage and becoming in part cartilage bones. Its anterior 

 portion is formed by a palatine bone which articulates with the 

 lateral ethmoid, and the middle portion by a pterygoid and a 

 mesopterygoid bone, while the hinder part is ossified above as a 

 metapterygoid and below as a quadrate. The latter articulates with 

 the lower jaw. Functionally, however, the primitive upper jaw 

 is now replaced by a secondary upper jaw, formed on each side 

 by a series of tooth-bones, situated external to the former, and 

 meeting in front of the prenasal region of the primary cranium 

 (Fig. 123, A). The series includes a dentigerous premaxilla and 

 maxilla, and a small toothless, scale-like jugal bone. Each half 

 of the lower jaw (Fig. 123, A, B) consists of a rod-like Meckel's 

 cartilage or primary lower jaw. The hinder part of this is 



