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-operculum_ 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. 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 {216}present in the Salmon. There is usually a median ossification of the ethmoid cartilage, the _mesethmoid_. An _entopterygoid_ is sometimes added to the palato-pterygo-quadrate series of bones. An ossification of the anterior extremity of each Meckelian cartilage may form a _mento-Meckelian_ bone. Certain additional membrane bones are sometimes developed in relation with the lower jaw, such as _splenial_ and _coronary_ bones on the inner side, and a _supra-angular_ bone at the angle of the jaw, above the angular element. To these there may be added the singular series of _infra-dentaries_, which in some fossil Crossopterygii (e.g. _Rhizodopsis_) fringe the outer margin of the jaw beneath the true dentary (Fig. 274, A). A system of _jugular plates_ may also form a characteristic armature for the throat between the lateral halves of the lower jaw (Fig. 274, C). Besides those already mentioned, additional sensory canal bones are present in some Fishes. A transverse row of plates (_supra-temporals_) sometimes crosses the occipital region behind the parietals. There are also other canal-ossicles which lose their identity by fusing with certain cranial or periotic bones. Thus, each of the pterotic and sphenotic bones often includes a superficial dermal bone transmitting a section of a sensory canal, which has fused with it; and as the frontal bone is often similarly perforated, it may be taken that it also includes a canal-ossicle; and the same can often be said of the articular and dentary bones of the lower jaw.[191]