328 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



attaching the jaw to the skull, that attachment being 

 effected solely by mandibular elements, and being, 

 therefore, autostylic. 



In the branchiate Yertebrata the number of 

 branchial arches corresponds with that of the 

 branchise, and the separate bars become segmented ; 

 all the visceral bars save the mandibular have a 

 distinct median basal piece, which is known as 

 the basibraiicliial ; this passes on either side 

 into the liypobraiicliial, which is succeeded 

 by the ceratobraiichial, epibrancliial, and 

 pharyiigobrancliial. When gills cease to be de- 

 veloped these bars undergo, as may be supposed, a 

 certain amount of atrophy, but, in all, the first 

 branchial arch is retained, while in tortoises and 

 lizards two arches may be detected. These basal 

 portions always fuse with those of the hyoid arch, and 

 the coalesced pieces make up the so-called body of the 

 hyoid, which forms a support for the tongue ; the 

 parts of the true hyoid arch form the interior, and 

 those of the first branchial the posterior or lesser 

 cornua of the hyoid of man. 



We have hitherto regarded the skull as compounded 

 of neural, sensory, and visceral portions, all of which 

 are formed by cartilage ; we have now to look at the 

 same structure from another point of view. It has 

 already been pointed out, that while the cartilage in 

 the occipital region of the skull forms a complete ring- 

 in its hinder portion, the sphenoidai region is roofed 

 in by membrane ; this membranous roof is retained 

 throughout life by Myxine (the hag). In other 

 Cyclostomes and in Elasmobranchs the roof becomes 

 more or less completely cartilaginous, and this carti- 

 lage, which never becomes ossified, though its outer 

 layers may be calcified, is covered in by membrane. 



In the more shark-like Ganoidei, the membrane, 

 though not the cartilage, undergoes ossification, and a 



