Before proceeding to explain the characters of the different groups, according to the modern system of classification, it may be desirable to explain the terms by which the different parts and characters are described, and to shew the manner in which the shells are measured. For this purpose we shall treat of the general divisions separately. We begin with UNIVALVE SHELLS. In considering Univalves merely with reference to their mathematical construction, the first point demanding our attention is, whether they are symmetrical or non-symmetrical, or, in other words, whether a straight line drawn through the shell would divide it into two equal parts. The greater part of univalves are non-symmetrical, being rolled obliquely on the axis; but many are symmetrical, being rolled horizontally on the axis. The Nautilus presents an illustration of the latter; the Snail is a familiar example of the former. _Symmetrical Univalves._ [Illustration] In describing these it will be well to commence with the most simple form, such as the Patella,--taking a conical species as an example. In this it will be observed that there is no winding or curvature, but a simple depressed cone, and that the line _a_, _p_, divides it into two equal parts. The _anterior_, _a_, (_cut_, fig. 10) is known by the interruption of the muscular impression which surrounds the central disc (_d._) This interruption of the muscular impression is in the place where the head of the animal lies in the shell. The impression itself is caused by the fibrous muscle which attaches the animal to the shell. The apex (_a_) in Patella, generally leans towards the anterior (_a_) part of the shell, and away from the posterior (_p_); and this circumstance has caused some mistakes, because in Emarginula the apex leans towards the posterior; and students, instead of examining the muscular impression, which is the only criterion, have only noticed the direction in which the apex turned, and concluded that to be the anterior, towards which it inclined. The lines or ribs running from the base to the apex of the shell, in the direction _r_, are called _radiating_ lines; and those which encircle the cone in the direction _c c_, from front to back, are very properly described as _concentric_. The _length_ is measured from front to back in the line _e_; the breadth, from side to side, in the line _b_; and the depth from the apex to the base.