VIII SKULL 215 
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. 125, 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 operculwm above, which articulates with a 
backwardly projecting process from the hyomandibular, followed 
in succession below by a sub-operculwm and an inter-operculum, 
the latter being connected by ligament with the angle of the 
lower jaw. The series is completed by ten sabre-shaped branchio- 
stegal 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. 125, A), and 
by one or two small bones situated above and on the outer side 
of each periotic capsule (sguamosals). 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 
