436 THE DOGFISH CHAP, x 



and lower jaws and supporting the 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 carti- 

 laginous 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 articu- 

 lation with the first vertebra. On the roof of the skull, 

 between and behind the olfactory capsules, there is a fon- 

 tanelle (p. 43), closed over by connective-tissue only ; and 

 between the two auditory capsules is a depression into 

 which open the two endolymphatic ducts of the ears 

 ' (p. 187). 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. /, /. /) are connected 

 with the cranium by ligaments (Ig, Ig) only, and each consists 

 of strong, paired (right and left) moieties, united with one 

 a other by fibrous tissue. The upper jaw, which corre- 



