According to Carpenter, when a fragment of any recent shell is decalcified by being placed in dilute acid, a definite animal basis remains, often so fine as to be no more than a membranous film, but sometimes consisting of an aggregation of ‘cells’ with perfectly definite forms. He accordingly divides all shell structure into _cellular_ and _membranous_, according to the characteristics of the animal basis. Cellular structure is comparatively rare; it occurs most notably in _Pinna_, where the shell is composed of a vast multitude of tolerably regular hexagonal prisms (Fig. 162 B). Membranous structure comprises all forms of shell which do not present a cellular tissue. Carpenter held that the membrane itself was at one time a constituent part of the mantle of the mollusc, the carbonate of lime being secreted in minute ‘cells’ on its surface, and afterwards spreading over the subjacent membrane through the bursting of the cells. The iridescence of _nacreous_ shells is due to a peculiar lineation of their surface, which can be readily detected by a lens. According to Brewster, the iridescence is due to the alternation of layers of granular carbonate of lime and of a very thin organic membrane, the layers very slightly undulating. Carpenter, on the other hand, holds that it depends upon the disposition of a single membranous layer in folds or plaits, which lie more or less obliquely to the general surface, so that their edges show as lines. The nacreous type of shell occurs largely among those Mollusca which, from other details in their organisation, are known to represent very ancient forms (_e.g._ _Nucula_, _Avicula_, _Trigonia_, _Nautilus_). It is also the least permanent, and thus in some strata we find that only casts of the nacreous shells remain, while those of different constitution are preserved entire. _Porcellanous_ shells (of which the great majority of Gasteropoda are instances) usually consist of three layers, each of which is composed of a number of adjacent plates, like cards on edge. The inclination of the plates in the different layers varies, but that of the plates in the inner and outer layer is frequently the same, thus if the plates are transverse in the middle stratum, they are longitudinal in the inner and outer strata, and, if longitudinal in the middle, they are transverse in the other two. Not uncommonly (Fig. 163 B) other layers occur. In bivalves the disposition and nature of the layers is much more varied. [Illustration: FIG. 162.--=A=, Section of shell of _Unio_: _a_, periostracal layer; _b_, prismatic layer; _c_, nacreous layer. =B=, Horizontal section of shell of _Pinna_, showing the hexagonal prisms.]