A Conchological Manual Author:George Brettingham Sowerby Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. Excerpt from book: Section 3In the former, represented by the cut, fig. 8, the tendon passes through a perforation in the upper valve; and in the latter, Mytilus, fig. 9, the byssus passes out between the valves. ... more »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. textit{Symmetrical Univalves. 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 thereis no winding or curvature, but a simple depressed cone, and that the line textit{a, p, divides it into two equal parts. The textit{anterior, «, textit{(cut, fig. 10) is known by the interruption of the muscular impression which surrounds the central disc ((/.) 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 textit{(a) in Patella, generally leans tow...« less