Fig. 260.—An extinct Crossopterygian, Holoptychius giganteus Agassiz (1835). (After Agassiz, per Zittel.)
The Actinopteri.—From the Crossopterygians springs the main branch of the true fishes, known collectively as Actinopteri, or ray-fins, those with ordinary rays on the paired fins instead of the jointed archipterygium. The transitional series of primitive Actinopteri are usually known as Ganoids. The Ganoid differs from the Crossopterygian in having the basal elements of the paired fins small and concealed within the flesh. But other associated characters of the Crossopterygii and Dipnoans are preserved in most of the species. Among these are the mailed head and body, the heterocercal tail, the cellular air-bladder, the presence of valves in the arterial bulb, the presence of a spiral valve in the intestine and of a chiasma in the optic nerves. All these characters are found in the earlier types so far as is known, and all are more or less completely lost or altered in the teleosts or bony fishes. Among these early types is every variety of form, some of them being almost as long as deep, others arrow-shaped, and every intermediate form being represented. An offshoot from this line is the bowfin (Amia calva), among the Ganoids the closest living ally of the bony fishes, showing distinct affinities with the great group to which the herring and salmon belong. Near relatives of the bowfin flourished in the Mesozoic, among them some with a forked tail, and some with a very long one. From Ganoids of this type the vast majority of recent fishes may be descended.
Fig. 261.—An ancient Ganoid fish, Platysomus gibbosus Blainville. Family Platysomidæ. (After Woodward.)
Fig. 262.—A living Ganoid fish, the Short-nosed Gar, Lepisosteus platystomus Rafinesque. Lake Erie.
Another branch of Ganoids, divergent from both garfish and bowfin and not recently from the same primitive stock, included the sturgeons (Acipenser, Scaphirhynchus, Kessleria) and the paddle-fishes (Polyodon and Psephurus). All these are regarded by Woodward as degenerate descendants of the earliest Ganoids, Palæoniscidæ, of Devonian and Carboniferous time.
Fig. 263.—A primitive Ganoid fish, Palæoniscum macropomum (Agassiz), restored. Permian. Family Potaconiscidæ. (After Traquair.)