The earliest ichthyc remains are of this fragmentary character. They have been obtained from the "Ludlow rock," a member of the upper Silurian series, and consist of spines and portions of skin, that have been thickly covered with hard tubercles and prickles, like the shagrcen of the shark's skin. The spines most nearly resemble the dorsal spine of the dogfish; they are small, flattened, and slightly curved. Along with other similar fragmentary remains, they have been placed under the somewhat indefinite generic,title onchus.
The minute, compressed, conical, and glistening bodies, called conodonts, obtained in great numbers from the lower Silurian measures in Russia, and considered by their describer. Pander, to have been the teeth of F. belong certainly to very differ ent animals. Their small size and peculiar forms, and the entire margin of the hol low base by which they were attached, show them to have been the denticles from the lingual ribbon of shell-less mollusks, which have left no other traces of their existence than these remarkable conodonts.
The Ludlow bone-bed contains the earliest noticed fish remains. No idea of the numerical importance of F. at this early period can be satisfactorily formed; yet these remains being confined to a single thin bed, and occurring rarely even in that, would seem to indicate that the Silurian seas were but thinly tenanted by these earliest sharks.
In the immediately succeeding Devonian rocks, their numbers largely increased: The ichthyodorulites, or fossil spines of this period, have been referred to fourteen differ ent genera. Numerous species of true ganoids lave been determined from their well preserved enamel scales, which occur singly or in confused groups, and frequently also associated with the head, fins, and tail, so as to present a faithful "nature-print " of the fish upon the rock. _See DIPTERUS, DIPLACANTHUS, etc. But the most remarkable and
characteristic fossils of this period are the buckler-fishes, whose head and part of their body were covered with bony plates, giving them so singular and anomalous an appear ance, that some of them were originally considered crustacean. They are almost con fined to the old red sandstone series, a single species (found in penman strata) being the only cephalaspid that is known later. See CEPIIALASPIS, CECCOSTEUS, PTERICII THYS, etc.
Fish remains are of frequent occurrence in the coal-measures. Upwards of twenty species of plagiostomous F. have been determined from the spine defenses, some of which are very large and powerful. The frequency with which the peculiar teeth of the cestracionts are met, show that they must have been common in the carboniferous seas. Ganoids were also abundant. See PALYEONISCUS, DOLOPTYCRIUS, etc.
In the permian period, the forms are similar to what exist in the older strata. Up to the last permian deposit, the fish have all possessed heterocercal tails; but with the secondary rocks, the homocercal tail not only appears, but becomes the more frequent form.
Numerous species and many new forms appear in the trias and oolite. Sharks are remarkably abundant in the cretaceous strata; but the chalk is specially remarkable from containing the earliest discovered remains of the true bone-fishes—those covered with ctenoid and cycloid scales..
In the tertiary strata, the character and proportion of ichthyc remains exhibit a con dition in the inhabitants of the water very similar to what at present prevails. The car tilaginous orders decrease, and are replaced by osseous F., such as the salmon, cod, tur bot, and herring—F. which are of much greater value to man than those they superseded.