Page:Popular Science Monthly Volume 12.djvu/534

516 known, although in the same formation of Europe Ganoids are abundant; and with them are remains of sharks, and some other fishes, the affinities of which are doubtful. The Palæozoic fishes at present known from this country are quite as numerous as those found in Europe.

In the Mesozoic age, the fishes of America begin to show a decided approach to those of our present waters. From the Triassic rocks, Ganoids only are known, and they are all more or less closely related to the modern gar-pike, or Lepidosteus. They are of small size, and the number of individuals preserved is very large. The characteristic genera are—Catopterus, Ischypterus, Ptycholepis, Rhabdolepis, and Turseodus. From the Jurassic deposit no remains of fishes are known, but in the Cretaceous, ichthyic life assumed many and various forms; and the first representatives of the Teleosts, or bony fishes, the characteristic fishes of to-day, make their appearance. In the deep open sea of this age, Elasmobranchs were the prevailing forms, Sharks and Chimæroids being most numerous. In the great inland Cretaceous sea of North America, true osseous fishes were most abundant, and among them were some of carnivorous habits and immense size. The more sheltered bays and rivers were shared by the Ganoids and Teleosts, as their remains testify. The more common genera of Cretaceous Elasmobranchs were—Otodus, Oxyrhina, Galeocerdo, Lamna, and Ptychodus. Among the osseous fishes, Beryx, Enchodus, Portheus, and Saurocephalus, were especially common, while the most important genus of Ganoids was Lepidotus.

The Tertiary fishes are nearly all of modern types, and from the beginning of this period there was comparatively little change. In the marine beds, Sharks, Rays, and Chimæroids, maintained their supremacy, although Teleosts were abundant, and many of them of large size. The Ganoids were comparatively few in number. In the earliest Eocene fresh-water deposits, it is interesting to find that the modern Gar-pike, and Amia, the dogfish of our Western lakes, which by their structure are seen to be remnants of a very early type, are well represented by species so closely allied to them that only an anatomist could separate the ancient from the modern. In the succeeding beds, these fishes are still abundant, and with them are Siluroids nearly related to the modern catfish (Pimelodus). Many small fishes, allied apparently to the modern herring (Clupea), left their remains in great numbers in the same deposits, and, with them has been recently found a landlocked ray (Heliobatis).

The almost total absence of remains of fishes from the Miocene lake-basins of the West is a remarkable fact, and perhaps may best be explained by the theory that these inland waters, like many of the smaller lakes in the same region to-day, were so impregnated with mineral matters as to render the existence of vertebrate life in them