SKELETON 



2,7 



parison of the lateral cranial bars 

 Avith the trabeculae, originally 

 made by Agassiz [4], is doubtless 

 well founded. Whilst Miiller saw 

 the mandibular arch represented 

 in the subocular bar, and the 

 hyoid in the styloid process, 

 cornual, and median ventral styli- 

 form cartilages, Huxley [229] 

 considered the latter to be man- 

 dibular, and the lingual to belong 

 to the hyoid arch. The annular 

 and corresponding cartilages in 

 Mijxine are generally compared 

 to the labials, and the subocular 

 arch to the pterygo-quadrate. 

 But branches of the trigeminal 

 nerve pass below the subocular 

 arch, not above as they should 

 if this arch were homologous with 

 the pterygo-quadrate bar (Figs. 

 20,21,22). It must be confessed 

 that the exact homology of these 

 cartilages is at present impossible 

 to determine, and that many of 

 them may be new formations in 

 the Cyclostome head. 



To understand the character 

 of the head skeleton other systems 

 must be taken into consideration. 

 It has already been shown that 

 in Petromyzon the first meta- 

 otic somite, corresponding to the 

 glossopharyngeal nerve, develops 

 the first permanent myotome of 

 the adult (p. 5). The muscles 

 of the head are therefore less 

 specialised than in other Crani- 

 ates. Moreover, not only does 

 the notochord extend fully into 

 the base of the cranium, but 

 there is no sort of articulation 

 between the hind part of the 

 skull and the anterior region of 

 the vertebral column. More im- 

 portant still : whereas in all other 



