SKELETON. 



157 



of a complicated cartilaginous framework, the vertical bars being 

 united by horizontal rods. There occur in connection with the 

 cranium several cartilaginous bars, while in front a series of 

 plates extend to the end of the head. There is no structure 

 comparable beyond a doubt to a hyoid ; while instead of mova- 

 bly articulated jaws, the mouth is supported by a cartilaginous 

 ring, and a well-developed cartilaginous framework exists in the 

 tongue, while the filaments around the mouth {Myxine) have 

 cartilaginous supports. 



In the elasmobranchs the skull is never converted into bone, 

 although calcareous deposits may be formed in its wall. The 

 cranium is a closed cap- 

 sule, sometimes carti- 

 laginous throughout, 

 sometimes with places 

 in its roof(fontanelles), 

 which are not chondri- 

 fied, but are closed with 

 membrane. Through 

 the walls are openings 

 for the passage of 

 nerves and blood-ves- 

 sels, but there is no 

 trace of division into 

 separate elements. 

 The pterygoquadrate 



in the normal sharks is united to the chondrocranium by liga- 

 ments and muscles, and by the hyomandibular suspensor. In 

 the holocephali, on the other hand, pterygoquadrate and cra- 

 nium are firmly anchylosed in the adult (Fig. 166), although 

 free in the young. 



Above the elasmobranchs bones appear in the skull, both as 

 ossifications of cartilage and as membrane bones. The more 

 constant and more important of these are as follows : 



The chondrocranium gives rise to four bones around the 

 large opening (foramen magnum) through which the brain is 

 connected with the spinal cord. These are, below, the basi- 

 occipital ; on either side an exoccipital ; and above, part of a 



FIG. 166. 



Skull of Chimara monstrosa (drawn 

 from a dry specimen). 



