The articulata of the Silurian period bore a still more peculiar character. They consisted mainly of the Trilobites--a family in whose nicely jointed shells the armorer of the Middle Ages might have found almost all the contrivances of his craft anticipated, with not a few besides which he had failed to discover; and which, after receiving so immense a development during the middle and later times of the Silurian period that whole rocks were formed almost exclusively of their remains, gradually died out in the times of the Old Red Sandstone, and disappeared forever from creation after the Carboniferous Limestone had been deposited. The mollusca of the Silurians ranged from the high cephalopoda, represented in our existing seas by the nautili and the cuttle-fishes, to the low brachiopods, some of whose cogeners may still be detected in the terebratulæ of the Highland lochs and bays, and some in the lingulæ of the Southern Hemisphere. The cephalopods of the system are all of an obsolete type, that disappeared myriads of ages ago. At length, in an upper bed of the system, immediately under the base of the Old Red Sandstone, the remains of the earliest known fishes appear, blended with what also appears for the first time--the fragmentary remains of a terrestrial vegetation. The rocks beneath this ancient bone-bed have yielded no trace of any plant higher than the Thallogens, or at least not higher than the Zosteraceæ--plants whose proper habitat is the sea; but, through an apparently simultaneous advance of the two kingdoms, animal and vegetable--though, of course, the simultaneousness may be but merely apparent--the first land-plants and the first vertebrates appear together in the same deposit. The earliest fishes--first-born of their family--seem to have been all placoids. The Silurian System has not yet afforded trace of any other vertebral animal. With the Old Red Sandstone the ganoids were ushered upon the scene in amazing abundance; and for untold ages, comprising mayhap millions of years, the entire ichthyic class consisted, so far as is yet known, of but these two orders. During the times of the Old Red Sandstone, of the Carboniferous, of the Permian, of the Triassic, and of the Oolitic Systems, all fishes, though apparently as numerous individually as they are now, were comprised in the ganoidal and placoidal orders. The period of these orders seems to have been nearly correspondent with the reign, in the vegetable kingdom, of the Acrogens and Gymnogens, with the intermediate classes, their allies. At length, during the ages of the Chalk, the Cycloids and Ctenoids were ushered in, and were gradually developed in creation until the human period, in which they seem to have reached their culminating point, and now many times exceed in number and importance all other fishes. The delicate Salmonidæ and the Pleuronectidæ--families to which the salmon and turbot belong--were ushered into being as early