FIG. 1.—Shoulder-girdle of a Flounder, _Paralichthys californicus_ (Ayres). ]
=The Series Ganoidei.=—Among the lower _Actinopteri_ many archaic traits still persist, and in its earlier representatives the group approaches closely to the _Crossopterygii_, although no forms actually intermediate are known either living or fossil. The great group of _Actinopteri_ may be divided into two series or subclasses, the _Ganoidei_, or _Chrondrostei_, containing those forms, mostly extinct, which retain archaic traits of one sort or another, and the _Teleostei_, or bony fishes, in which most of the primitive characters have disappeared. Doubtless all of the _Teleostei_ are descended from a ganoid ancestry.
Even among the _Ganoidei_, as the term is here restricted, there remains a very great variety of form and structure. The fossil and existing forms do not form continuous series, but represent the tips and remains of many diverging branches perhaps from some Crossopterygian central stock. The group constitutes at least three distinct orders and, as a whole, does not admit of perfect definition. In most but not all of the species the tail is distinctly and obviously heterocercal, the lack of symmetry of the tail in some Teleosts being confined to the bones and not evident without dissection. Most of the Ganoids have the skeleton still cartilaginous, and in some it remains in a very primitive condition. Usually the Ganoids have an armature of bony plates, diamond-shaped, with an enamel like that developed on the teeth. In all of them the pectoral fin has numerous basal bones or actinosts. All of them have the air-bladder highly developed, usually cellular and functional as a lung, but connecting with the dorsal side of the gullet, not with the ventral side as in the Dipnoans. In all living forms there is a more or less perfect optic chiasma. These ancient forms retain also the many valves of the arterial bulb and the spiral valve of the intestines found in the more archaic types of fishes. But traces of some or all of these structures are found in some bony fishes, and their presence in the Ganoids by no means justifies the union of the Ganoids with the sharks, Dipnoans, and Crossopterygians to form a great primary class, _Palæichthyes_, as proposed by Dr. Günther. Almost every form of body may be found among the Ganoids. In the Mesozoic seas these fishes were scarcely less varied and perhaps scarcely less abundant than the Teleosts in the seas of to-day. They far exceed the Crossopterygians in number and variety of forms. Transitional forms connecting the two groups are thus far not recognized. So far as fossils show, the characteristic actinopterous fin with its reduced and altered basal bones appeared at once without intervening gradations.