VIII 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 le in the roof of the mouth. The 
vomer is a tooth-bone, and probably the parasphenoid also. 
The mandibular arch (Fig. 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 
the cranium of Salmo 
salar, showing the right 
half of the cranial 
cavity. Cartilage is 
dotted. 7, Frontal ; v’, 
fat-containing cavity in 
the mesethmoid carti- 
= ‘ lage ; V, WII, IX, X, 
v aan! : foramina for the fifth, 
as oe ined seventh, ninth, and 
tenth cranial nerves. Remaining reference-letters as in Fig. 121. (From W.K. Parker.) 
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 
mesopterygovd 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. 125, A). The series includes a dentigerous premazxilla 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 
