The two strata thus formed, one richly coloured, the other white, often nacreous and brilliantly iridescent, are highly organized substances. Examined with the microscope, they present remarkable varieties in some of the natural groups of bivalve Mollusca; the structure of the Monomyarian Oyster is characteristic of the division which has but one muscle; the Dimyaria, having two muscles, are represented by the Cockle. [Illustration: Fig. 167. Section of shell of Pinna transversely to the direction of its prisms.] [Illustration: Fig. 168. Membranous basis of the shell of Pinna.] The exterior laminæ at the edge of the fragile valves of a Pinna are often so thin and transparent that the organization of the shells may be seen with a low magnifying power. A fragment has the appearance of a honeycomb on both surfaces (fig. 167), whereas its broken edge resembles an assemblage of basaltic columns. The exterior layer of the shell is thus composed of a vast number of nearly uniform prisms, usually approaching to the hexagonal structure, whose lengths form the thickness of the lamina, their extremities its surfaces. When the calcareous part of the lamina is dissolved by dilute acid, a firm membrane is left, which exhibits a hexagonal structure (fig. 168), as in the original shell; but it is only in the shells of a few families of bivalves nearly allied to the Pinna that this combination of the organic and mineral elements is seen in this distinct form; it is beautifully displayed in the nacreous shells. [Illustration: Fig. 169. Section of nacreous lining of the shell of Avicula margaritacea (pearl oyster).] In many shells the internal layer has a nacreous or iridescent lustre, shown by Sir David Brewster to depend upon the striation of its surface, by a series of nearly parallel grooved lines. When Dr. Carpenter had dissolved the calcareous matter from a thin piece of nacreous substance, taken from the shell of the Haliotis splendens, or Ear Shell, there remained an iridescent membrane, which presented to the microscope a series of folds or plaits somewhat regular, and splendidly iridescent, but when the plaits were unfolded and the membrane stretched, the iridescence vanished. So the varied hues of mother-of-pearl are owing to the folds of an organic membrane.