The bony fish (Osteon = "bone"; "icthys" = "fish") are the most diverse and numerous of all vertebrates. Bony fish (Class Osteichthyes) are first seen in fossils from the Devonian (about 395 million years before present). They differ from most of the cartilaginous fishes in having a terminal mouth and a flap (operculum) covering the gills. In addition, most have a swim bladder, which is ordinarily used to adjust their buoyancy, although among the air-breathing fishes it is attached to the pharynx and serves as a simple lung. The skin has many mucus glands and is usually adorned with dermal scales. Their jaws are well developed, articulated with the skull, and armed with teeth. Although the skeleton of most is bone, that of sturgeons and a few others is largely made of cartilage. They have a two-chambered heart built on the same plan as the Chondrichthyes (two-chambered with a conus arteriosus and a sinus venosus). The sexes are separate, most are oviparous, and fertilization is usually external. There are two subclasses: subclass Actinopterygii (ray-finned fishes) and subclass Sarcopterygii (lobe-finned fishes).
Sarcopterygians have a fleshy lobe at the base of their fins that is leg-like in appearance. They include fossil forms that are ancestors of the amphibians, the true lungfishes, and the coelacanth. Today's coelacanths are "living fossils," represented by a single species (Latimeria) found near the coast of Madagascar. Until 1938, when a coelacanth was first captured by chance, they were known only from the fossil record. Since then several dozen have been captured and some of their behavior has been filmed using robotic cameras. Latimeria is important because it provides an opportunity to compare observations from the fossil record with a living animal.
The Actinopterygii is the larger of the two subclasses. These animals have slender fin rays suporting their fins and lack the odd appendages of the lobe-finned fish. Most actinopterygians have a symmetrical caudal fin (homocercal tail) and a swim bladder. This group includes most of the fishes with which you are familiar (bass, goldfish, guppies, sea horses, sturgeons, and tuna). Most of the specimens you will see belong to the superorder Teleostei. Teleosts are the most successful of the fishes (they make up 95% of all fish species). They are also the most modern of the fishes (they evolved about 65 million years ago).
The only surviving holosteans are garpikes (Lepisoste) and bowfins (Amia). Members of the third superorder (Chondrostei) are common in the fossil record, but are represented today only by the freshwater sturgeons, bichirs, paddlefishes, and reedfishes.
Ganoid scales are covered with a thick enameloid called ganoine which lies on a thick support of lamellar bonewith a layer of dentine sandwiched between the two. Cycloid scales are recognized by their thinner structure and smooth edges. Also note the growth rings (circuli) radiating from the central (the "bull's-eye" near the center). Ctenoid scales are also thin compared to the ganoid scales and can be recognized by their serrated edge (caused by extending from the focus to the anterior margin of the animal).
The bony scales of a teleost are covered by a thin epidermal layer, within which are found the globular mucous glands. The bases of the scales are inserted into a layer of fibrous connective tissue. The fibrous layers and scales below the epidermis make up the dermal layer of the skin. The subcutis layer and muscle are located beneath the dermis. Fish scales are not plugged into skin like your hair (if you pull a hair out you don't destroy an overlying epidermis).