Several membrane bones are commonly formed around the orbit or cavity for the eye. The most constant of these is the lachrymal which lies in the anterior corner; frequently too, as in Teleosteans, there is a supra-orbital lying in the upper part of the orbit, or as in many Reptiles, a postorbital lying in the posterior part of the orbit.
(c) Nasal capsule.
In relation to the nasal capsules various bones occur.
The basicranial axis in front of the presphenoid is ossified, as the mesethmoid, dorsal to which there sometimes, as in Teleosteans, occur a median ethmoid and a pair of lateral ethmoids[9]. Two pairs of membrane bones very commonly occur in this region, viz. the nasals which lie dorsal to the mesethmoid, and the vomers (sometimes there is only one) which lie ventral to it.
The part of the skull lying immediately in front of the cranial cavity and in relation to the nasal capsules constitutes the ethmoidal region.
There remain certain other membrane bones which are often found connected with the cranium. Of these, one of the largest is the parasphenoid which, in Ichthyopsids, is found underlying the basicranial axis. Prefrontals often, as in most reptiles, occur lying partly at the sides and partly in front of the frontal, and postfrontals similarly occur behind the orbit lying partly behind the frontals and partly at their sides. Lastly a squamosal bone is, as in Mammals, very commonly developed, and lies external and partly dorsal to the auditory capsules.
The Jaws and Visceral Skeleton.
In the most primitive fish these consist of a series of cartilaginous rings or arches placed one behind another and encircling the anterior end of the alimentary canal. Originally they are mainly concerned with branchial respiration.
The first or maxillo-mandibular arch forms the upper jaw and the lower jaw or mandible.
The second or hyoid arch bears gills and often assists in attaching the jaws to the cranium. The remaining arches may bear gills, though the last is commonly without them.