INTRODUCTION. 13 



In 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. 



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. 

 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 



