The depth of this group, as estimated by Mr. Murchison, is equal to double the height of our highest Scottish mountains; and four distinct platforms of being range in it, the one over the other, like stories in a building. Life abounded on all these platforms, and in shapes the most wonderful. The peculiar encrinites of the group rose in miniature forests, and spread forth their sentient petals by millions and tens of millions amid the waters; vast ridges of corals peopled by their innumerable builders,--numbers without number,--rose high amid the shallows; the chambered shells had become abundant--the simpler testacea still more so; extinct forms of the graptolite, or sea-pen, existed by myriads; and the formation had a class of creatures in advance of the many-legged annelids of the other. It had its numerous family of trilobites,--crustaceans nearly as high in the scale as the common crab,--creatures with crescent-shaped heads, and jointed bodies, and wonderfully constructed eyes, which, like the eyes of the bee and the butterfly, had the cornea cut into facets resembling those of a multiplying glass. Is the reader acquainted with the form of the common _Chiton_ of our shores--the little boat-shaped shell-fish, that adheres to stones and rocks like the limpet, but which differs from every variety of limpet, inbearing as its covering a jointed, not a continuous shell? Suppose a chiton with two of its terminal joints cut away, and a single plate of much the same shape and size, but with two eyes near the centre, substituted instead, and the animal, in form at least, would be no longer a chiton, but a trilobite. There are appearances, too, which lead to the inference that the habits of the two families, though representing different orders of being, may not have been very unlike. The chiton attaches itself to the rock by a muscular sucker or foot, which, extending vent rally along its entire length, resembles that of the slug or the snail, and enables it to crawl like them, but still more slowly, by a succession of adhesions. The locomotive powers of the trilobite seem to have been little superior to those of the chiton. If furnished with legs at all, it must have been with soft rudimentary membranaceous legs, little fitted for walking with; and it seems quite as probable, from the peculiarly shaped under margin of its shell, formed, like that of the chiton, for adhering to flat surfaces, that, like the slug and the snail, it was unfurnished with legs of any kind, and crept on the abdomen. The vast conglomerations of trilobites for which the Silurian rocks are remarkable, are regarded as further evidence of a sedentary condition, Like _Ostreæ_, _Chitones_, and other sedentary animals, they seemed to have adhered together in vast clusters, trilobite over trilobite, in the hollows of submarine precipices, or on the flat, muddy bottom below. And such were the master existences of three of the four Silurian platforms, and of the greater part of