The curtain rises, and the scene is new. The myriads of the lower formation have disappeared, and we are surrounded, on an upper platform, by the existences of a later creation. There is sea all around, as before; and we find beneath a dark-colored, muddy bottom, thickly covered by a dwarf vegetation. The circumstances diner little from those in which the ichthyolite beds of the preceding period were deposited; but forms of life, essentially different, career through the green depths, or creep over the ooze. Shoals of _Cephalaspides_, with their broad, arrow-like heads, and their slender, angular bodies, feathered with fins, sweep past like clouds of crossbow bolts in an ancient battle. We see the distant gleam of scales, but the forms are indistinct and dim: we can merely ascertain that the fins are elevated by spines of various shape and pattern; that of some the coats glitter with enamel; and that others--the sharks of this ancient period--bristle over with minute thorny points. A huge crustacean, of uncouth proportions, stalks over the weedy bottom, or burrows in the hollows of the banks. Let us attempt bringing our knowledge of the present to bear upon the past. The larger crustaceæ of the British seas abound most on iron-bound coasts, where they find sheltering places in the deeper fissures of sea-cliffs covered up by kelp and tangle, or under the lower edges of detached boulders, that rest unequally on uneven platforms of rock, amid forests of the rough-stemmed cuvy. We may traverse sandy or muddy shores for miles together, without rinding a single crab, unless a belt of pebbles lines the upper zone of beach, where the forked and serrated fuci first appear, or a few weed-covered fragments of rock here and there occur in groups on the lower zones. In this formation, however, the bottom must have been formed of mingled sand and mud, and yet the crustacea were abundant. How account for the fact? There is, in most instances, an interesting conformity between the character of the ancient rocks, in which we find groups of peculiar fossils, and the habitats of those existences of the present creation which these fossils most resemble. The fisherman casts his nets in a central hollow of the Moray Frith, about thirty fathoms in depth, and draws them up foul with masses of a fetid mud, charged with multitudes of that curious purple-colored zoophyte the sea-pen, invariably an inhabitant of such recesses. The graptolite of the most ancient fossiliferous rocks, an existence of unequivocally the same type, occurs in greatest abundance in a finely-levigated mudstone, for it, too, was a dweller in the mud. In like manner, we may find the ancient Modiola of the Lias in habitats analogous to those of its modern representative the muscle, and the encrinite of the Mountain Limestone fast rooted to its rocky platform, just as we may see the Helianthoida and Ascidioida of our seas fixed to their boulders and rocky skerries. But is not analogy