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.
External surface. Fig. 91, cancellated; 92, coronated; 93, imbricated; 94, pectinated; 95, plicated; 96, decussated; 97, muricated; 98, foliated.
By the foregoing general observations and explanations, it is trusted that the reader will be prepared for the following exposition of the general arrangement of Lamarck, and the principles upon which it is founded.
GENERAL ARRANGEMENT
ACCORDING TO LAMARCK.