The direction in which the successive layers are deposited, forms another ground of distinction. The disc is formed in some instances of a series of whorls, the apex or nucleus being more or less central; if these whorls are numerous, the operculum is described as _multispiral_, as in shells of the genus Trochus; if few, as in Cyclostoma, it is _paucispiral_. In some instances the flattened spire consists of but one whorl, it is then _unispiral_; and when scarcely one turn is completed, it is described as _subspiral_. When the layers are applied upon each other in such a manner that the nucleus is central, and the edges of the subsequent layers are extended beyond each other all around, so as to form rims, the operculum is described as _concentric_; if the nucleus is lateral, or at one side without being spiral, it is _lammellated_; and when it forms a terminal point, enlarging in the form of a finger-nail or claw, it is _unguiculated_. In the operculum of a Neritina, there is a lateral process, by means of which it is locked into the columella, the term _articulated_ is then applied. In that of Navicella, there is also a process which appears to radiate from the nucleus, it has therefore been described as a _radiated_ operculum. _Opercula of Spiral Univalves._ [Illustration: Fig. 64, _multispiral_; 65, _paucispiral_; 66, _concentric_; 67, _articulated_; 68, _radiated_; 69, _lammellated_; 70, _unguiculated_.] BIVALVE SHELLS. Bivalve shells, named Conchacea by Lamarck, are those which consist of two principal portions united to and folded upon each other by means of a hinge. The pieces united compose the shell, while each piece separately is called a valve. Considering the bivalve shell as a whole, it will be necessary, in the first instance, to describe the position in which it is to be observed, in order to give the student a clearly defined notion of what is intended, when terms expressive of height, depth, length, breadth, &c. are used, as well as when the anterior and posterior extremities are spoken of. For this purpose, we must suppose the animal to be living and creeping along the bed of the sea by means of its foot; where this foot protrudes, will be the _ventral margin_, and the opposite part the _dorsal margin_ of the shell. There will then be a valve on each side; and if we further suppose the animal to be walking forward with its back to the observer, the _right_ and _left_ valves will correspond with his right and left sides. _Measurements._ [Illustration] The _length_ will be measured from _anterior_ (_a_) to _posterior_ (_p_), and the lines of growth running in the same direction will consequently be _longitudinal_ or _concentric_; _transverse_ of some authors. The height will be from the umbones (_u_), to the _ventral margin_, and lines or bands in that direction are termed _radiating_; longitudinal, according to some authors.