GENERAL CHARACTERS. 



901 



(fig. 820). The quadrate gives off an anterior palatopterygoid (pa/a- 

 toquadrate) bar, which in Sharks (fig. 822) persists to form a kind 

 of false upper jaw. The hyoid arch, which is close to the mandi- 

 bular, and which in Fishes may also take a share in the support of 

 the mandible, is likewise segmented ; the most important elements 

 in the latter class being the hyomandibular and the symplectic, which 

 are shown in an ossified condition in fig. 823, hm, sy. As already 

 mentioned, in Sharks the primitive cartilaginous skull is complete, 

 and persists in this condition throughout life ; but in the great 

 majority of Vertebrates the progress of chondrification is arrested, 



Fig. 823. — Left lateral view of the suspensorial and mandibular region of the skull of a Salmon. 

 hm, Hyomandibular ; mt, Metapterygoid ; qn, Quadrate ; sy, Symplectic ; po, Preopercular ; 

 io, Interopercular ; so, Subopercular ; o, Opercular ; ar, Articular ; d, Dentary. (After Parker.) 



and the skull becomes more or less completely covered in with a 

 series of ossifications developed in the membrane overlying the 

 brain, while the cartilaginous foundation itself is likewise converted 

 into bone. Bones derived from these two totally distinct sources 

 amalgamate in the adult in such a manner as to afford no clue to 

 their dual origins. Following a modification of an arrangement 

 adopted by Professor Weinsheimer, the more important bones of 

 the cranium may be enumerated as follows : the relative position of 

 most of them being shown in figs. 824 and 825. 1 The cartilage 

 bones comprise the basioccipital, basisphenoid, and presphenoid, 

 which are median ossifications (not shown in the two figures) lying 

 in this order (from the posterior extremity) on the inferior aspect 

 of the cranium, and the first forming the floor of the foramen mag- 

 num, or aperture by which the spinal cord enters the cranium, and 

 the three being collectively known as the basi-c?-anial axis. On 



1 In the sequel the figures of the skulls of many reptiles, and more especially 

 those of Ichthyosaurus and Nothosauras, show the general relations of the bones 

 of the skull to advantage. 



