In some species the layers are irregularly grouped together, and their edges overlap each other, so that they are easily separable, and advancing beyond each other, give a leafy appearance to the external surface. This structure is termed _foliaceous_. A very familiar instance of this may be observed in the common oyster. If a specimen of this shell be broken, the substance will be seen to exhibit a degree of looseness, and a magnifying glass will enable the student to trace distinctly the laminæ of which it is composed. The accompanying representation of a magnified section (fig. 6) will shew at _a_, the external surface, with the foliations or leaves; at _b_, the parcels of layers which form them; and at _c_, the pearly structure produced by their consolidation, and by the subsequently deposited enamel which covers their external surface. [Illustration: 6. Section of an oyster shell enlarged.] CLASSIFICATION. The classification of shells, that is, their systematic arrangement into _classes_, _orders_, _families_, _genera_ and _species_, cannot be made to depend entirely upon the characters observable in them, viewed by themselves; for this reason, that many similarly formed shells form the habitations of animals perfectly distinct, and that many molluscous animals are found to agree with each other in every respect but in the form of their testaceous support. There are, however, many important distinctions to be observed in the shells themselves, leading to the establishment of many of those very divisions, which would afterwards be confirmed by an examination of the soft parts. It is necessary to attend, as far as means and opportunity will allow, to _all_ the points of difference, both in the shell and in the animal, in order to form, and in some instances even to appreciate, a generic or larger distinction. It will therefore be our endeavour to explain the general principles upon which those distinctions are formed, and the manner in which they are applied and expressed in detail by scientific writers. NUMBER OF PIECES, OR INDEPENDENTLY FORMED PARTS. The first, most simple and obvious division of shells, is that which results from the number of separate pieces composing them. Hence the distinction implied by the terms UNIVALVE, or consisting of a single piece; BIVALVE, or composed of two pieces; and MULTIVALVE, or composed of more than two. For an example of _univalve_, take a common whelk; for a _bivalve_, take a muscle or a scallop; and for a _multivalve_, the barnacle, or balanus, found adhering to the common oyster.