From the three families last named, with the _Pholidophoridæ_, there is an almost perfect transition from the Ganoid fishes to teleosteans of the order of _Isospondyli_, the primitive order from which all other bony fishes are perhaps descended. The family of _Leptolepidæ_, differing from _Oligopleuridæ_ in the absence of fulcra, is here placed with the _Isospondyli_, but it might about as well be regarded as Ganoid. CHAPTER III ISOSPONDYLI =THE Subclass Teleostei, or Bony Fishes.=—The fishes which still remain for discussion constitute the great subclass or series of _Teleostei_ (τελεός, true; οστέον, bone), or bony fishes. They lack wholly or partly the Ganoid traits, or show them only in the embryo. The tail is slightly, if at all, heterocercal; the actinosts of the pectoral fins are few and large, rarely over five in number, except among the eels; the fulcra disappear; the air-bladder is no longer cellular, except in very rare cases, nor does it assist in respiration. The optic nerves are separate, one running to each eye without crossing; the skeleton is almost entirely bony, the notochord usually disappearing entirely with age; the valves in the arterial bulb are reduced in number, and the spiral valve of the intestines disappears. Traces of each of the Ganoid traits may persist somewhere in some group, but as a whole we see a distinct specialization and a distinct movement toward the fish type, with the loss of characters distinctive of sharks, Dipnoans, and Ganoids. In a general way the skeleton of all Teleosts corresponds with that of the striped bass (see Figs. 22, 23, Vol. I), and the visceral anatomy is in all cases sufficiently like that of the sunfish (Fig. 16, Vol. I). The mesocoracoid or precoracoid arch, found in all Ganoids, persists in the less specialized types of bony fishes, although no trace of it is found in the perch-like forms. With all this, there is developed among the bony fishes an infinite variety in details of structure. For this reason the _Teleostei_ must be broken into many orders, and these orders are very different in value and in degrees of distinctness, the various groups being joined by numerous and puzzling intergradations.