The Crustacea, so called from the hard crust or shell which envelops them, form, with all their orders and genera, a very numerous family. They are of interest to us as containing among their number some of the oldest forms of life. Away down in the lower Silurian rocks, among the most ancient fossiliferous strata, we find the crustacean with its armour of plates and its prominent sessile eyes set round with lenses, still visible on the stone. Thus, on the first page of the stony records of our planet's history are these primeval organisms engraved. In some localities, where oxide of iron is largely present, they are coated with a bright yellow efflorescence, and stand out from the dull grey stone like figures embossed in gold.[31] On all the subsequent leaves of this ancient chronicle, we can detect the remains of crustacean life, and many tribes still swarm in our seas and lakes. It is interesting, however, as marking the onward progress of creation, to notice that, though this great family has continued to live during all the successive geological ages, its members have ever been changing, the older types waning and dying out, while newer genera rose to supply for a time their place, and then passed away before the advance of other and yet later forms. The trilobites that meet us on the very verge of creation, swarmed by millions in the seas of the Silurian ages, diminished gradually during the era of the Old Red Sandstone, and seem to have died out altogether in the times of the Coal. In no ocean of the present day is a trace of any of their many genera to be seen. The _decapods_, of which our common crab is a typical form, began to be after the trilobites had died out. In all the subsequent eras they gradually increased in numbers, and at the present day they form the most abundant order of crustacean life. The history of these two divisions, to adopt Agassiz's mode of representation, may be illustrated by two long tapering bands like two attenuated pyramids. The one has its broad base resting upon the existing now, and thinning away into the past, till at last it comes to a point. At a little interval the apex of the other begins, and gradually swells outward as it recedes, till the wide base terminates at the first beginnings of life. [Footnote 31: Such is the aspect of the organisms in some of the Silurian sandstones near Girvan. I have seen the same bright tint on a set of fossils from the Llandeilo flags of Wales, and from the slates of Desertcreat, Ireland, and have disinterred similarly gilded shells from the vertical greywackè slates of the Pentland Hills and Peeblesshire. Nothing can be more beautiful than the aspect of these fossils when first laid open, but the bright gleam eventually passes away on exposure.]