MORPHOLOGY OF TELEOSTEAN HEAD SKELETON. 513 



relative to the rest of the plate, and dips down somewhat 

 below the level of the flat basis cranii which the remainder of 

 the parachordal forms with its fellow. 



The occipital section has now sent up on either side a stout 

 occipital arch (figs. 2 and 1 , oc. a.). On the outer side and near 

 the base of this is a slight projection almost like a transverse 

 process [oc. a.'). In front of the arch lies the exit of the vagus 

 nerve (IX and X), whilst all the so-called occipital nerves 

 pass out entirely behind it. 



The anterior and external wall of the auditory capsule {au. c.) 

 has extended backwards, underneath, and along the outer 

 side of the aud,itory vesicle to the level of the occiput ; but in 

 the cranium figured has no connection with this. In that of 

 a slightly older larva, it is continuous both with the lateral 

 process of the occipital arch {oc. a.') and with the postero- 

 lateral border of the mesotic I'egion, thus forming a complete 

 boundary around the exit for the ninth and tenth nerves (IX, 

 X). In this larva, also, there are two fenestras in the floor of 

 the auditory capsule (cp. fig. 2, /e.') separated by a bridge of 

 cai'tilage which is incomplete in the younger one. This 

 bridge is formed in correlation with a slight elevation of the 

 floor of the auditory capsule. This elevation is more strongly 

 marked posteriorly by the presence of a pillar of cartilage 

 {au. c.'), around which the external semicircular canal runs. 

 The whole floor of the capsule fits the bottom of the auditory 

 labyrinth, and seems to have been moulded to it, so that where 

 this bulges the cartilage is thinned out, and fenestras {fe.') 

 are left; where it is constricted a cartilaginous ridge is formed. 



The anterior edge of the auditory capsule is greatly 

 thickened. The pro-cartilaginous post-orbital process (fig. 6, 

 sh. p.), which has elongated and become more completely 

 chondrified (fig. 7), arises from the dorsal end of this thicken- 

 ing; whilst a similar but more delicate process (o. pr.'), lying 

 between the exits of the fifth and seventh nerves, projects 

 from the ventral end. This thickening is undoubtedly to be 

 compared with the alisphenoid region of the salmon, where it 

 arises as a " growth of cartilage downwards from the (supra- 



