VIII SKULL 2 1 5 



ensheathed on its outer side by a large tooth-bone, the denti- 

 gerous dentary. The hyoid arch is similar to that of the Dog- 

 Fish, except that its primitively cartilaginous segments are almost 

 completely ossified (Fig. 123, B). The large upper segment or 

 hyomandibular bone articulates mainly with the pterotic, but 

 partly also with the sphenotic element of the periotic capsule ; 

 below it is connected with a slender symplectic bone, and from 

 the cartilage connecting the two depends the rest of the hyoid 

 arch, consisting in succession of stylo-hyal, epi-hyal, cerato-hyal, 

 and hypo-hyal bones, with a median teeth-bearing basi-hyal. The 

 palato-pterygo-quadrate bar has no direct connexion with the 

 skull, except anteriorly where its palatine element articulates 

 with the lateral ethmoid. The real suspensorium is formed by 

 the hyomandibular and symplectic bones, to which the hinder 

 margins of the quadrate and metapterygoid bones are rigidly 

 attached by suture, hence, as in the Dog-Fish, the skull is 

 hyostylic. Behind the hyoid arch there are five branchial 

 arches, which generally resemble those of the Dog-Fish, except 

 that their component segments are ossified as cartilage bones. 



Connected with the hyomandibular and cerato-hyal elements 

 of the hyoid arch there is, on each side, a series of membrane 

 bones for the support of the movable operculum or gill-cover. 

 These consist of an operculum above, which articulates with a 

 backwardly projecting process from the hyomandibular, followed 

 in succession below by a sub-operculuvi and an inter-opemdum, 

 the latter being connected by ligament with the angle of the 

 lower jaw. The series is completed by ten sabre-shaped branchio- 

 stecfcd rays, which are attached to the cerato-hyal and support the 

 lower margin of the gill-cover. 



Sensory canal bones are represented in the Salmon by a ring 

 of small bony plates which encircle the orbit (Fig. 123, A), and 

 by one or two small bones situated above and on the outer side 

 of each periotic capsule {squamosals). To these may be added 

 the pre-operculum situated external to the hinder margins of the 

 hyomandibular and quadrate bones, firmly clamping these bones 

 together, and also the post-temporals, by which the secondary 

 pectoral girdle is attached to the skull. The nasal bones may 

 also be regarded as pertaining to the same series. 



In other Fishes with a more or less complete bony skull there 

 are certain additional cartilage- and membrane-bones which are not 



