_Granular_, or, as they are sometimes called, _Concretionary_ shells, are the most hard and compact; it is in these that the substance called _nacre_, or _mother-of-pearl_, is mostly found. One of the commonest examples is the oyster shell; if broken across it will be seen to consist of very thin plates, or _laminæ_, as they are termed, closely packed together. The thinner these laminæ may be the more lustrous and beautiful appears the lining of the shell; that shifting play of colours which we call _iridescent_, from iris—the rainbow, is then brightest and most noticeable. A very remarkable substance is this mother-of-pearl; smooth, and shining, and delicately-tinted. Who would expect to find such a beautiful lining to the rugged, rough, dingy-looking oyster, or mussel shell? Truly these mollusks, some of them, live in gorgeous palaces. And the most curious part of the matter is that from the fluids or juices, of their own bodies, and from the chalky matter collected from the water, they are enabled to secrete or deposit such wonderfully-constructed habitations, which after all are little more than chalk. Burn a heap of oyster shells, or any other testaceous coverings, and you get lime the same as that produced by burning the white lumps from the chalk-pit, which lumps, by the way, are said to be composed wholly, or for the most part, of marine shells. This we should call _cretaceous_ matter, from _creta_, which is the Latin for chalk, or _calcerous_, from _calcis_—lime. Granular shells you have been told are sometimes called _concretionary_, this is because they contain a large amount of this chalky deposit. The rock called limestone, geologists tell us, is composed entirely of fossil shells and mud, or what was once mud, dried and hardened, most likely by extreme heat, to the consistence of rock. Wonderful this to think of; huge mountains, and mighty masses, and far-stretching strata, forming a large portion of the crust of the earth, made up chiefly of the coverings of fishes, a great portion of them so small as to be scarcely visible to the naked eye.—Truly wonderful! But we shall have more to say upon this head when we come to speak of Fossil Shells, as well as on the subject of Pearls, in our chapter on the fish in whose shells they are chiefly found.