The fin contains about ten soft rays.
The hind-limb lies immediately behind the median union of the pectoral arch. The basal part of the limb consists of a long triangular bone produced by the contact of a right and left element, which appear to be homologous with the two metapterygia (right and left), i. e. the posterior basalia of the hind-limb in an Elasmobranch. The true pelvis is therefore absent. Teleostei and Ganoidei, with the exception of Polypterus, which has small pelvic cartilages, agree in this respect, and differ from Elasmobranchii, Holocephali, and Dipnoi, which possess a pelvis. The fin-rays are articulated to the posterior and external faces of this double bone. The first ray in each fin is entire, the remaining five are soft.
The following additional points may be noted relative to the skull. A series of suborbital bones lies beneath the eye of which the most anterior is exceedingly large. These bones are removed in this specimen. In some Teleostei, e.g. the Trout, there are similar bones above the eye. The optic nerve passes out of the cranium between the arms of the basisphenoid. The bone termed alisphenoid is continued forwards for a short distance by membrane which is pierced by the olfactory nerve. The two nerves separated by the interorbital septum traverse the back of the orbit and pierce the ectoethmoids on the way to the olfactory mucous membrane. An 'ocular' canal which lodges the recti muscles of the eyes passes backwards into the forepart of the basi-occipital, beneath the pro-ouc and above the parasphenoid. The inner wall of the ear-capsule is replaced by thin membrane which is easily injured in dissection, and the vertical semi-circular canals then appear to lie in the cranial cavity.
The skulls of Teleostei in general agree closely with that of the Perch. The size of the cartilaginous cranium varies, as may be seen on comparison of a Salmon with the Perch. The amount of persistent cartilage as compared with bone also varies. Anchylosis of the bones may take place. In Cyprinoid and Siluroid fish there is no inter-orbital septal plate, but there are orbito- and pre-sphenoid ossifications in this region. In the Pike the mesethmoidal cartilage is partially ossified by two pairs of bones instead of a median one, and in the Salmon this region is not ossified but is covered by a supra-ethmoid bony plate. The Pike has a small supra-orbital bone. Praemaxillae are absent in Muraenoids. In some Teleostei, e.g. Pike, a small bone placed distally on the posterior margin of the maxilla appears to represent the jugal. The metapterygoid is absent in the Siluroid Clarias capensis. And in Siluroids generally the sub-opercular is absent. In Cyprinoids the fifth branchial arch is strongly bowed, and carries prominent teeth which work against the horny basi-occipital tooth.
The group of Pharyngognathi is so named from the fact that the fifth pair of arches is fused into a single dentigerous plate.
When the lower jaw is connected to the cranium solely by a hyomandibular element derived from the hyoid arch as it is in Teleostei, in Ganoidei except Lepi-dosteus, in the majority of Elasmobranchii, it is said to be hyostylic. When it is connected not only by a hyoidean element but also by a quadrate, as in Lepidosteus, or by a palatoquadrate, as in Cestracion among Elasmobranchii and probably in Holocephdli, it is said to be amphistylic; and when it is connected by a quadrate element alone as in Amphibia and Sauropsida, it is termed autostylic. In Mammalia the dentary element articulates with the squamosal, the articular portion of the lower jaw and the quadrate having been converted into ear-bones, i. e. malleus and incus respectively.
The azygos system of fins appears in Teleostei as in other fish as a continuous fold of skin supported by embryonic fin-rays which are afterwards replaced by permanent fin-rays. Such a continuous fold extending from the back of the head round the tip of the tail to the anus persists in Blennies, Eels, Congers, Soles, Ophididae, etc., among Teleostei, and in Dipnoi. But this condition in the opinion of the late Professor Balfour has been secondarily acquired. The caudal fin is first differentiated from the continuous fold: then the posterior dorsal when present, unless the anterior dorsal is of a peculiar type, as in Lophius, when it appears before the posterior. The anal fin appears before the pelvic fins, unless the latter are of a peculiar type and adapted to special uses, as in the young of some Gadoidei.
The young Elasmobranch, Lepidosteus, and Teleostean, have at first a long pointed tail, to the tip of which the notochord extends and the lateral line as well whenever this primitive condition is retained. A caudal fin is next developed on the ventral aspect and at some little distance from the extremity of this pointed prolongation as an enlargement of the continuous fold. The prolongation bends upwards towards the dorsal aspect at the same time. The growing caudal fin, as remarked by Professor A. Agassiz, has much the appearance of a second or posterior anal. It is supported solely by enlarged haemal arches, beyond which appear at a later period the fin-rays. Such a condition persists and forms the heterocercal caudal fin of all Elasmobranchii, living chondrostean Ganoids, and many extinct Ganoids. But in existing bony Ganoids and the Teleostei the pointed prolongation atrophies until the caudal fin becomes terminal. The upward dorsal inclination is preserved in the urostyle. The part of the caudal fin formed on the neural side of the urostyle is always inconsiderable. This outwardly symmetrical caudal fin, really asymmetrical and, anatomically speaking, heterocercal, is termed homocercal.
But in a few fish, such as the Dipnoi and the Teleostei above-mentioned, the backbone retains its straight course and divides the caudal fin into two equal portions, dorsal and ventral. Such fins are known as diphycercal. In the Eel and some other eel-like Teleosteans, rudimentary haemal arches exist and point to the existence at some distant period of a caudal lobe now aborted: and in them the diphycercal tail is secondarily acquired. In Holocephali the long whip-like tail has the groove of the lateral line continued to its apex, and a small ventral lobe represents the large caudal lobe of Elasmobranchii. The late Professor Balfour believed that he had found traces of caudal haemal arches in Cera-todus, which would indicate a lost caudal lobe in that Dipnoan. It is perhaps doubtful whether a primary diphycercal tail exists among living Pisces.