It is not unlikely that this process of moulting takes place annually in most of the Crustacea, so that if we suppose a fossil member of the group to have lived six years, it would have left six crusts to be entombed in any deposits that might be forming at the time. Of course there would be many chances against all the six being preserved, but the possibility of at least several of them becoming fossilized should be borne in mind when we speculate on the abundance of such organisms in any geological formation. I might refer to another very interesting group of crustacean animals known as the _Limuli_, or king-crabs, of which there were at least three representatives during the times of the English Carboniferous system (Fig. 20-3). They are remarkable chiefly for their large crescent-shaped shield, their long sword-like tail, and their double pair of eyes, of which the outer ones are large, sessile, and compound, like those of the trilobites, while the middle pair are small, simple, and set close together on the forehead, like those of the single-eyed Cyclops in the old mythology. Altogether, with their shields, swords, watchful waking eyes, strong massive armour, and great size (for some of them measure two feet in length), they form a most warlike genus. CHAPTER VI. Carboniferous fauna continued--George Herbert's ode on "Man"--His idea of creation--What nature teaches on this subject--Molluscous animals--Range of species in time proportionate to their distribution in space--Two principles of renovation and decay exhibited alike in the physical world and the world of life--Their effects--The mollusca--Abundantly represented in the carboniferous rocks--Pteropods--Brachiopods--Productus--Its alliance with Spirifer--Spirifer--Terebratula--Lamellibranchs--Gastropods-- Land-snail of Nova Scotia--Cephalopods--Structure of orthoceras-- Habits of living nautilus. Holy George Herbert, in one of the most remarkable odes of the seventeenth century, sang quaintly, yet nobly, of the dignity of man. He looked into the design and nature of the human heart, and saw there a palace that had been built for the abode of the Eternal. Deserted though it might be, broken down and in ruins, yet there still lingered a trace of its ancient glory, and the whole material world still testified to its inherent greatness. He looked abroad on the face of nature, and saw, in all its objects and all its movements, a continued ministration to man. "For us the windes do blow; The earth doth rest, heav'n move, and fountains flow. Nothing we see, but means our good, As our delight, or as our treasure; The whole is, either our cupboard of food, Or cabinet of pleasure.