The name _Ganoidei_ (γάνος, brightness; εἶδος, resemblance), alluding to the enameled plates, was first given by Agassiz to those forms, mostly extinct, which were covered with bony scales or hard plates of one sort or another. As the term was originally defined, mailed catfishes, sea-horses, _Agonidæ_, _Arthrodires_, _Ostracophores_, and other wholly unrelated types were included with the garpikes and sturgeons as Ganoids. Most of these intruding forms among living fishes were eliminated by Johannes Müller, who recognized the various archaic characters common to the existing forms after the removal of the mailed Teleosts. Still later Huxley separated the Crossopterygians as a distinct group, while others have shown that the _Ostracophori_ and _Arthrodira_ should be placed far from the garpike in systematic classification. Cope, Woodward, Hay, and others have dropped the name Ganoid altogether as productive of confusion through the many meanings attached to it. Others have kept it as a convenient group name for the orders of archaic _Actinopteri_. For these varied and more or less divergent forms it seems convenient to retain it. As an adjective "ganoid" is sometimes used as descriptive of bony plates or enameled scales, some-in the sense of archaic, as applied to fishes.
=Are the Ganoids a Natural Group?=—Several writers have urged that the _Ganoidei_, even as thus restricted, should not be considered as a natural group, whether subclass, order, or group of orders. The reasons for this view in brief are the following:
1. The group is heterogeneous. The _Amiidæ_ differ more from the other Ganoids than they do from the herring-like Teleosts. The garpikes, sturgeons, paddle-fishes likewise diverge widely from each other and from the _Palæoniscidæ_ and the _Platysomidæ_. Each of the living families represents the residue or culmination of a long series, in some cases advancing, as in the case of the bowfin, sometimes perhaps degenerating, as in the case of the sturgeons.
2. Of the traits possessed in common by these forms, several (the cellular air-bladder, the many valves in the heart, the spiral valve in the intestine, the heterocercal tail) are all possessed in greater or less degree by certain _Isospondyli_ or allies of the herring. All these characters are still better developed in _Crossoptergyii_ and _Dipneusti_, and each one disappears by degrees. Of the characters drawn from the soft parts we can know nothing so far as the extinct Ganoids are concerned.