42 INTRODUCTION. 



tinguished 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 striae 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 pro- 

 jecting 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 fold- 

 ings in their surface, as shewn in cut, fig. 95. A strong 

 instance of this is seen in the Ostraea 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 num- 

 ber 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 



