THE SKULL 



285 



common to find that some of the rectus eye-muscles pass back 

 into a tunnel beneath the brain-case ; the so-called eye- 

 muscle-canal or myodome. 



In the most primitive amphibia, the membrane-bones also 

 make a complete covering to the skull, for which reason these 

 animals are called Stegocephalia. Many of these bones can 

 be identified with those of Osteolepid fish because they are 



*pm. 



pc.^ 



Fig. 



132. — Stegocephalian (Loxomma) : dorsal view of a skull, showing 

 the course of the lateral-line canals. (Drawn from a cast.) 



grooved by the lateral- line system. The only openings in the 

 roof of the skull in the Stegocephalia are the nostrils, the orbits, 

 and the median pineal foramen which in these animals lies 

 between the parietal bones. The spiracles are, of course, 

 closed in land-vertebrates, but the position of their former 

 openings is indicated by a notch in the hind border of the roof 

 on each side. 



In land-vertebrates the skull and vertebral column are 



