Page:Popular Science Monthly Volume 60.djvu/570

562 from which diverge radius and ulna, the carpal bones being formed of the intervening cartilage.

From the crossopterygians springs the main branch of true fishes, known collectively as Actinopteri, those with ordinary rays on the paired fins instead of the jointed archipterygium. The transitional series of primitive Actinopteri is here called by the name of Ganoid. The ganoid differs from the Crossopterygian in having basal elements of the paired fins small and concealed within the flesh. But other associated characters of the Crossopterygii and Dipnoi 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 known, and all are more or less completely lost or altered in the teleosts or bony fishes. Among the existing ganoids the gar-pike (Lepisosteus) is the last of a long series of Mesozoic forms of the same general structure. The gar-pikes are cylindrical or arrow-shaped. Among these early types is every variety of form, some of them being almost as long as deep, and every intermediate form being represented. An offshoot from this line is the bow-fin (Amia calva), perhaps 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 bow-fin fiourished in the Mesozoic, among them some with a forked tail, and some with a very long one. From forms of this type the body of recent fishes may be descended.

Another branch of ganoids, widely divergent from both gar-fish and bow-fin and not recently from the same primitive stock, included the sturgeons (Acipenser, Scaphirhynchus, Kessleria) and the paddle-fishes (Polyodon and Psephurus). These differ widely from any other types, recent or fossil, showing features of degeneration as compared with their extinct ancestors, while again sturgeon and paddlefish differ widely from, each other. It has been suggested that the cat-fishes (Siluridæ) are descended from the sturgeons, but the resemblance vanishes the more closely the groups are compared, nor are we anywhere sure of the point where any part of the teleost series is joined to the ganoids. We can only say that the sturgeons are more or less degraded ganoids with cartilaginous skeletons, of unknown derivation and of unsettled relationships.

All other fishes have ossified instead of cartilaginous skeletons. The dipnoan and ganoid traits one by one are more or less completely lost. Through these the main line of fish development continues and the various groups are known collectively as bony fishes or teleosts.

The earliest of the true bony fishes or teleosts appear in Mesozoic times, the most primitive forms being soft-rayed fishes with