This order includes the great majority of fishes in which there is a well-ossified endoskeleton, and it corresponds very nearly with Cuvier's division of the "osseous" fishes. The Teleostei are defined as follows :-The skeleton is usually well ossified ; the cranium is provided with cranial bones; and a mandible is present; whilst the vertebral column almost always consists of more or less completely ossified vertebra. The pectoral arch has a clavicle; and the two pairs of limbs, when present, are in the form of fins supported by rays. The gills are free, pectinated or tufted in shape ; a bony gill-cover and branchiostegal rays being always developed. The branchial artery has its base developed into a bulbus arteriosus; but this is never rhythmically contractile, and is separated from the ventricle by no more than a single row of valves.

The order Teleostei comprises almost all the common fishes; and it will be unnecessary to dilate upon their structure, as they were taken as the types of the class in giving a general description of the Fishes. It may be as well, however, to recapitulate very briefly some of the leading characters of the order.

I. The skeleton, instead of remaining throughout life more or less completely cartilaginous, is now always more or less thoroughly ossified. The notochord is not persistent, and the vertebral column, though sometimes cartilaginous, consists of a number of vertebrae. The bodies of the vertebras are what is called "amphiccelous" - that is to say, they are concave at both ends. It follows from this, that between each pair of vertebrae there is formed a doubly-conical cavity, and this is filled with the cartilaginous or semi-gelatinous remains of the notochord. By this means an extraordinary amount of flexi-bility is given to the entire vertebral column. In no fish except the Bony Pike (which belongs to the order Ganoidei) is the ossification of the vertebral centra carried further than this. The skull is of an extremely complicated nature, being composed of a number of distinct cranial bones; and a mandible or lower jaw is invariably present.

II. The anterior and posterior pairs of limbs are usually, but not always, present, and when developed they are always in the form of fins. The fins may be supported by "spinous" or "soft" rays, of which the former are simple undivided spines of bone, whilst the latter are divided transversely into a number of short transverse pieces, and also are broken up into a number of longitudinal rays proceeding from a common root. (The fishes with soft rays in their paired fins are termed" Malacopterygii" - those with spinous rays, "Acan-thopterygii")

III. Besides the paired fins, representing the limbs, there is a variable number of unpaired or azygous integumentary expansions, which are known as the "median fins." When fully developed (fig. 261), they consist of one, two, or three fins on the back - the "dorsal" fins; one or two on the ventral surface - the "anal" fins; and one clothing the posterior extremity of the body - the "caudal" fin. The caudal fin (fig. 253, A) is set vertically, and not horizontally, as in the Whales and Dolphins; and in all the bony fishes its form is "homocercal" - that is, it consists of two equal lobes, and the vertebral column is not prolonged into the superior lobe.* In all the median fins the fin-rays are supported upon a series of dagger-shaped bones, which are plunged in the flesh of the middle line of the body, and are attached to the spinous processes of the vertebrae. These are the so-called "interspinous " bones.

* Though to all appearance symmetrical, the tail of the bony fishes is in reality generally unsymmetrical. The appearance of symmetry is due to the bony spinal column terminating in the centre of a wedge-shaped

Fig. 261.   The common Perch (Perca fluviatilis). o Gill cover, with the gill slit behind it; p One of the pectoral fins, the left; v The left ventral fin ; d The first dorsal fin ; d' The second dorsal fin ; c The caudal fin or tail; a The anal fin; l Lateral line.

Fig. 261. - The common Perch (Perca fluviatilis). o Gill-cover, with the gill-slit behind it; p One of the pectoral fins, the left; v The left ventral fin ; d The first dorsal fin ; d' The second dorsal fin ; c The caudal fin or tail; a The anal fin; l Lateral line.

IV. The heart consists of two chambers, an auricle and a ventricle, and the branchial artery is furnished with a bulbus arteriosus. The arterial bulb, however, is not furnished with a special coat of striated muscular fibres, is not rhythmically contractile, and is separated from the ventricle by no more than a single row of valves.

V. The respiratory organs consist of free, pectinated, or tufted branchiae, situated in two branchial chambers, each of which communicates internally with the pharynx by a series of clefts, and opens externally on the side of the neck by a single aperture (or "gill-slit"), which is protected in front by a bony gill-cover (fig. 261) and is also closed by a "branchiostegal membrane," supported upon "branchiostegal rays." The branchiae are attached to a series of bony branchial arches (generally five on each side, but only the anterior four bearing gills), which are connected inferiorly with the hyoid bone, and superiorly with the skull; and the water required in respiration is taken in at the mouth by a process analogous to swallowing.

"hypural bone," to the free edges of which the caudal fin-rays are symmetrically attached. The actual termination of the notochord is bent up, and is never ossified ; but its sheath usually becomes calcified, forming a spine ("urostyle ") which coalesces with the dorsal edge of the hypural bone, the latter being formed by the anchylosis of ossicles developed from the ventral face of the notochordal sheath.

VI. The nasal sacs never communicate posteriorly with the cavity of the pharynx.

VII. The exoskeleton usually has the form of overlapping horny scales of the cycloid or ctenoid character; but it is sometimes absent, sometimes composed of scattered plates of true bone, sometimes ganoid, and sometimes formed of shagreen-like bony spines.

VIII. The stomach is capacious; pyloric caeca are present; the intestine has no spiral valve; and the rectum usually opens separate from and in front of the urinary and genital apertures. The air-bladder may or may not be present, and may or may not communicate with the gullet. The kidneys are well developed. The reproductive organs may be solid, and may liberate their contents by rupture into the abdominal cavity; but they are usually hollow organs, with ducts which open beside or behind the urinary aperture.

The subdivisions of the osseous fishes are so numerous, and they contain so many families, that it will be sufficient to run over the more important sub-orders, and to mention the more familiar examples of each.

Sub-order A. Malacopteri, Owen (= Physostomata, Mul-ler). - This sub-order is defined by usually possessing a complete set of fins, supported by rays, all of which are "soft" or many-jointed, with the occasional exception of the first rays in the dorsal and pectoral fins. A swim-bladder is always present, and always communicates with the oesophagus by means of a duct, which is the homologue of the windpipe. The skin is rarely naked, and is mostly furnished with cycloid scales; but in some cases ganoid plates are present.

This sub-order is one of great importance, as comprising many well-known and useful fishes. It is divided into two groups, according as ventral fins are present or not. In the first group - Apoda - there are no ventral fins; and the most familiar examples are the common Eels of our own country. The Eels (Muraenidae) have an elongated, almost cylindrical body, with the scales deeply sunk in the skin, and scarcely apparent. A swim-bladder is present, and the operculum is small and mostly enveloped in the skin. More remarkable, however, than the ordinary Eels is the Gymnotus electricus, or great Electric Eel (fig. 262), which inhabits the marshy waters of those wonderful South American plains, the so-called "Llanos," and which shares with various fishes of diverse affinities (the Torpedo, the Malapterurus electriais, etc.) the power of generating electricity by means of special organs.

The second group of the Malacopteri is that of the Abdominalia, in which there are ventral fins, and these are abdominal in position. Space will not permit of more here than merely mentioning that in this section are contained amongst others the well-known and important groups of the Clupeidae (Herring tribe), the Pikes (Esocidae), the Carps, Barbels, Roach, Chub, Minnow, etc. (Cyprinidae), and the Salmonidae, comprising the various species of Salmon and Trout. Also belonging to this group are the Sheat-fishes (Siluridae), which are chiefly noticeable because they are amongst the small number of living fishes possessed of structures of the same nature as the fossil spines known as "ichthyodorulites." The structure in question consists of the first ray of the pectoral fins, which is largely developed, and constitutes a formidable spine, which the animal can erect and depress at pleasure. Unlike the old "ichthyodorulites," however, the spines of the Siluridae have their bases modified for articulation with another bone, and they are not simply hollow and implanted in the flesh. The "Siluroids" are also remarkable for their resemblance to certain of the extinct Ganoid fishes (e.g., Pterichthys, Coccosteus, etc.), caused by the fact that the head is protected with an exoskeleton of dermal bones. The largest European species is the Silurus giants of the Swiss lakes, and of various European rivers. Another remarkable member of this family is the Malapterurus of the Nile and West Coast of Africa, which is endowed with electrical powers.

Fig. 262   Electric Eel (Gymnotus electrictts).

Fig. 262 - Electric Eel (Gymnotus electrictts).