SKULL. 



99 



otherwise membranous roof. In the floor there is a basi-cranial 

 fontanelle (Fig. 52, 9) in the anterior (trabecular) region just in 

 front of the anterior end of the notochord ; this transmits the 

 pituitary pouch or posterior nasal canal, which, passing back 

 from the nasal capsule, ends blindly in the Petromyzotitidae 

 (Fig. 51, 1), but opens into the mouth in the Myxinidae. This 

 canal lies between the basilar plate and the roof of the mouth. 

 The olfactory capsule is single and attached to the anterior part 

 of the cranium by fibrous tissue (Fig. 50, 9). The auditory 



Fig. 51. — Longitudinal vertical section through tlie anterior end of Petrotnyzon fluviatilis 

 (after Huxley) , 1 Blind end of posterior nasal canal ; 2 hinder margin of hard palate (inter- 

 trabecula) ; 3 cartilaginous roof of skull ; 4 brain ; 7 nasal capsule ; 8 posterior wall of 

 nasal capsule ; 9 the anterior portion of the subocular arcade ; 10 postero-lateral carti- 

 lage ; 11 postero-dorsal cartilage (ethmovomerine plate) ; 12 tongue ; 13 anterolateral 

 cartilage ; 14 anterodorsal cartilage ; 15 annular lip cartilage ; 16 median ventral carti- 

 lage ; 17 lingual cartilage; 18 ventral division of fifth nerve coming tlirough the sub- 

 ocular foramen ; 19 cornual cartilage ; 20 posterior part of subocular arch ; 21 styloid 

 process (hyoid) ; 22 tentaculate branchial valve ; 23 pharyngeal velum ; 24 internal 

 opening of first Ijranchial pouch ; 25 ditto of second branchial pouch ; 26 suboesophageal 

 tube ; 27 oesophagus ; 28 notochord ; 29 spinal cord. 



y. 



capsules (Fig. 50, 6) are attached laterally in the posterior region 

 on each side of the basilar plate. The subocular arch (Fig. 50, 17) 

 is a ventro-lateral continuation of the basilar plate and trabe- 

 cular region on each side ; it contains a fenestra and is supposed 

 to correspond to the subocular or palato-quadrate arcade of the 

 Amphibian skull. At the point where the posterior part of this 

 arch joins the basilar plate, there is given off ventralwards the 

 styloid process (Fig. 50, 18, and Fig. 51, 21), the end of which in 

 Petromyzon extends horizontally as the cornual cartilage (Fig. 

 50, 19). The styloid process and cornual cartilage have been 



