The _parietal_ valves, composing the principal part of the shell, vary in number, form and position. The _anterior_ valves are placed on the same side with the cirrhi; the _posterior_, those on the opposite side; and those which remain between on each side are the lateral valves. In many cases, particularly in Balanus, each valve is separated into the _prominent_ and _depressed_ areas, and the inserted lamina. In some instances, the parietal portion is formed by a single rounded piece. [Illustration] In the accompanying cut (87), the prominent areas are distinguished by the letters _pr_, and the depressed areas by _r_; the posterior valves of the operculum are marked _p. o._, and the anterior _a. o._ The basal valve (fig. 88) belongs to a Balanus. Fig. 89 is an Acasta, the cup-shaped base of which is represented at fig. 90. In the foregoing explanations we have omitted many of those general terms which, relating to external characters, are applicable to shells in almost every division of the system. It may be as well, however, to enumerate a few of them in this place, although they are explained under their respective letters in the alphabetical part of the work. When bars or ribs, or large striæ are crossed by others radiating from the umbones, shells are said to be _cancellated_, as represented in cut, fig. 91. When there is a series of nodules or spines on the upper part of the whorls, they are _coronated_, as shewn in cut, fig. 92. When a series of projecting parts overlay each other, in the manner of tiles, as in the cut, fig. 93, the word _imbricated_ is applied. When marked by a regular series of ridges, radiating from the apex, they are _pectinated_; the species of Chiton, a single valve of which is represented in cut, fig. 94, has received the specific name of _pectinatus_, in consequence of this character. Shells are said to be _plicated_ when characterized by angular bendings or foldings in their surface, as shewn in cut, fig. 95. A strong instance of this is seen in the Ostræa Crista-Galli. When the margin of any shell has a series of minute notches, resembling the teeth of a saw, it is said to be _serrated_; when covered with raised points or spines it is _aculeated_; and when striated in both directions, it is _decussated_; when covered with a number of raised rounded points, it is _granulated_; and having a series of these points placed in a row, near or upon the edge, it is _denticulated_, as already explained in reference to the outer lips of Spiral Univalves. When the external surface is rendered uneven by raised knobs, it is said to be _tuberculated_; and if rendered rough and prickly by sharp points it is _muricated_, as in the cut, fig. 97. The term _reticulated_ is applied to fine raised lines, crossing each other, and resembling fine net-work.