THE SKULL 283 



auditory capsule, but with regard to the jaws, there are three 

 types of attachment : 



Ampkistylic, as in the dogfish Hexanchus, and in Cladose- 

 lache. Here the upper jaw has an otic process which abuts 

 against the auditory capsule, and in addition the hyomandibula 

 serves to sling the upper jaw from the neurocranium. 



Hyostylic, as in Scyllium. The upper jaw nowhere touches 

 the auditory capsule, and is suspended by the hyomandibula, 

 and ligaments. 



Autostylic, as in Ceratodus and higher vertebrates. The 

 hyomandibula takes no share in the suspension of the upper 

 jaw, which is attached to the neurocranium by its own 

 processes. 



The processes of attachment of the upper jaw in autostylic 

 skulls are typically three in number. The otic process abuts 

 against the auditory capsule, and lies in front of the main 

 branch of the facial nerve ; the basal process abuts against the 

 floor of the neurocranium, and lies above and in front of the 

 palatine nerve (facial) ; the ascending process rises up on the 

 outside of the pila antotica with which it may or may not join, 

 and lies between the ophthalmic (Vi) and the maxillary (V2) 

 branches of the trigeminal nerves. 



The autostylic vertebrates above Ceratodus are terrestrial 

 animals which no longer use the gill-slits for respiratory 

 purposes in the adult. So the spiracular cleft gives rise to the 

 tympanic cavity and Eustachian tube, and the hyomandibula 

 becomes the columella auris. 



This description of the typical chondrocranium can be 

 applied to most groups of vertebrates. In the mammals an 

 important modification occurs in that the ascending process 

 comes to lie between the maxillary (V2) and mandibular (V3) 

 branches of the trigeminal nerve, and it is usually known as the 

 ala temporalis. 



Bony Skull. — Attention may now be turned to the bony 

 skull. The replacing, or cartilage-bones, are fairly constant 

 throughout the vertebrate series. In the neurocranium they 

 surround the brain, the olfactory and auditory capsules ; 

 while in the splanchnocranium they form the main skeletal 



