476 



MANDIBULAR AND IIYOID BARS. 



S.Or 



known, it is necessary to call attention to the mode of support of the 

 mandibular arch in certain forms, notably Notidanus, Hexanchus 

 and Cestracion, where the pterygo-quadrate region of the mandibular 

 arch is directly articulated to the cranium between the optic and tri- 

 geminal foramina. In the two former genera the metapterygoid region 

 of the arch is moreover continuous with the pterygo-quadrate, and 

 articulates with the post-orbital process of the auditory region of the 

 skull. In spite of these attachments the mandibular arch continues 

 to be partially supported by the hyomandibular. The skulls in which 

 the mandibular arch has this double form of support have been called 

 by Huxley amphistylic. 



Considering the in many respects primitive characters of the 



forms with amphistylic 

 skulls it seems not 

 improbable that they 

 preserve the origin-d 

 mode of support of the 

 mandibular arch ; from 

 which differentiations 

 in two directions have 

 taken place, viz. dif- 

 ferentiations in the 

 direction of a complete 

 support of the mandi- 

 bular arch by the hyoid, 

 which is characteristic 

 of most Elasmobranchii 

 and, as will be shewn 

 below, of Ganoidei and 

 Teleostei ; and differ- 

 entiations towards a 

 direct articulation or 

 attachment of the man- 

 dibular arch to the 

 cranium, without the 

 intervention of the 

 hyoid. The latter mode 

 of attachment is called 

 by Huxley autosty lie. 

 It is found in Holo- 

 cephala. Dipnoi, Amphibia and the Amniota. 



Teleostei. In addition to that of Elasmobranchii, the skull of the 

 Salmon is the only hyostylic skull in which, by the admirable investi- 

 gation of Parker (No. 45 1), the ontogeny of the hyoid and mandibular 

 bars has been satisfactorily worked out. Apart from the presence of 

 a series of membrane bones, the development of these bars agrees on 

 the whole with the types already described. 



The hyoid arch, though largely ossilied, undergoes a process of 



,Pa.cA. 



KM 



Fig. 334. Cranial skeleton of a Salmon fry, second 

 week after hatching ; membrane bones, eyeballs, and 

 NASAL SACS REMOVED. (Fiom Parker.) 



T.Cr. tegmen cranii ; S.Or. supraorbital band ; Fo. 

 superior fontanelle ; Av. auditory capsule ; Pa.cli. 

 parachordal cartilage ; Gli. notocbord ; Tr. trabecula ; 

 above the trabecula, the interorbital septum is seen, 

 passing into the cranial wall above and reaching the 

 supraorbital band ; //. optic foramen ; V. trigeminal 

 foramen ; V,IK labial cartilages ; Pl.Ft. palatopterygoid 

 bar ; M.Pt. metapterygoid tract ; Qa. quadrate region ; 

 Mck. Meckelian cartilage ; H.M. hyomandibular carti- 

 lage ; Sij. symplectic tract ; I.Hy. interhyal ; C.Hy. 

 ceratohyal ; H.i/y. hypohyal; G.ii?/. glossohyal ; Br.l. 

 lirst branchial arch. 



