The fish-remains next in age in America are from the Bloomfield sandstone in Pennsylvania, of the Onondaga Period in the Silurian; the earliest in Europe are found in the Ludlow shales, both of these being in or near the hori zon of the Niagara rocks, in the Upper Silu rian Age.
It is, however, certain that these Ordovician remains do not represent the beginning of fish life. Doubtless sharks, ostracophores and arthrodires, with perhaps, chimaeras, crossop terygians, and dipnoans existed at a far earlier period, possibly preceded by unarmed, limb less forms, without jaws, of which no trace whatever has been left.
Next appear more or less simultaneously, in the early or middle Silurian, fin-spines, thought to belong to primitive sharks, remains of acan thodean and other sharks and with these numer ous bony shields of the mailed ostracophores, and somewhat later those of the more highly specialized arthrodires. Later appear cestra mont sharks, chimaeras, dipnoans and cross opterygians.
In the Devonian Age the ostracophores in crease in size and abundance, disappearing with the beginning of the Carboniferous. The arthrodires also increase greatly in variety and in size, reaching their culmination in the Devonian, but not disappearing entirely until well in the Carboniferous.
These two groups (often but wrongly united by geologists under the older name 'placoderms, given by McCoy), together with sharks and chimeras, made up almost ex clusively the rich fish-fauna of Devonian times. The name ganoid has been associated with these forms but the true Ganoides are genuine fishes. The sharks were chiefly acae thodian and cladoselachean (Pleuropterygn), so far as our records show. The supposedly most primitive known type, that of Cladose lache, appeared in the middle Devonian. The Ichthyotomi (Pleuracanthus), sometimes re garded as still more primitive, are not known before the Carboniferous. In any case it is clear that the records of early shark-life are still incomplete. Chimmroids abound in Devonian, and with them a considerable variety of crossopterygians and dipnoans. The true fishes appear also in the Devonian,in the guise of ganoid ancestors and relatives of Palceoniscum. all these with diamond-shaped enameled scales. In the Devonian 'too, we find
the minute, forms (Pokrospon dyhts), our ignorance of which is concealed under the name Cyclic..
In the Carboniferous Age the sharks in crease in number and variety, the ostracophores disappear and the arthrodires follow them soon after, in the Permian. Other forms of dipnoans, crossopterygians and some ganoids follow, giving the fauna a somewhat more modern aspect The Acanthodei and Ich thyotomi pass away with the Permian, which follows the period of coal.
In the Triassic the earlier types of ganoids or mailed fishes of relatively modern type give place to forms approaching the gar-pike and sturgeon. The crossopterygians rapidly de dine. The dipnoans are less varied and fewer in number, and notidanoid sharks made their first appearance. All the ancient types of shark meanwhile have passed away, the Cestraciontes being the only group continued from the Permian to the Triassic. Here are found the first true bony fishes, derived from ganoid stock, the allies andpredecessors of the great group of herrings. These become more numerous in the Jura, and in this age appear other forms which give the fish-fauna of this period something of a modern appearance. In this age the sharks become divided into sev eral groups, Notidani, cat-sharks, lamnoid sharks, angel-fishes, skates and finally typical sharks, being well differentiated from each other. Chimaeras are still numerous. The Acanthodei have passed away, as well as the mailed ostracophores and arthrodires. The dipnoans and crossopterygians are few. The early ganoids have given place to more modern types still still in great abundance and variety.
This condition continues into the Cretaceous Period. Here the rays and modern sharks in crease in number: The ganoids hold their own, and other groups of soft-rayed fishes, as the smelts, the lantern-fishes, the pike and the fiy ing-fishes, join the group of herring-like forms which represent the modern bony fishes. In the Cretaceous appear the first spiny-rayed fishes, derived perhaps from pike-like forms, the ancestors of the living genus Beryx. From the berycoids spring the perch-like and mackerel-like forms so numerous in recent times.