436 THE DOGFISH CHAP, x 



gills ; and there are also skeletal parts in the median 

 fins. 



The cranium or brain-case (Fig. 112, Cr) is an irregular 

 cartilaginous box containing a spacious cavity for the 

 brain, very similar to the chondrocranium of the frog 

 (p. 43). It is produced into two pairs of outstanding 

 projections : a posterior pair, called the auditory capsules 

 (aud. cp), for the lodgment of the organs of hearing, 

 ridge-like projections on which indicate the position of 

 the semicircular canals (p. 187) ; and, in front of the 

 brain-cavity, an anterior pair, the olfactory capsules (olf. 

 cp), for the organs of smell, open below, and separated 

 from one another by a septum. Between the olfactory 

 and auditory capsules, on either side, the cranium is 

 hollowed out into an orbit (or), bounded by a supra- 

 orbital and a sub-orbital ridge, for the reception of the 

 eye. In front the brain-case is produced into three 

 cartilaginous rods, forming the rostrum (r) and supporting 

 the snout. On its posterior face is the foramen magnum 

 (p. 40), on either side of which is an oval condyle for 

 articulation with the first vertebra. On the roof of the 

 skull, between and behind the olfactory capsules, there 

 is a fontanelle (p. 43), closed over by connective-tissue 

 only ; and between the two auditory capsules is a depres- 

 sion into which open the two endolymphatic ducts of 

 the ears (p. 465). The nerve-apertures will be referred 

 to at a later stage. 



In the human and other higher vertebrate skulls the 

 upper jaw, as we have seen to be the case in the frog 

 (Fig. 9), is firmly united to the cranium, and the lower 

 alone is free. But in the dogfish both jaws (up, j, I. j) 

 are connected with the cranium by ligaments (Ig, lg') 

 only, and each consists of strong, paired (right and left) 

 moieties, united with one another by fibrous tissue. 

 The upper jaw, which corresponds to the palatoquadrate 



