INTRODUCTION. xxv 



is closed by a lid called the opercidum. It consists of 

 a plate composed in most cases of horny, but some- 

 times of calcareous matter, and it is attached to the 

 foot of the animal by a strong muscle. It is usually 

 spiral, but in a few instances it is concentric. 



The following woodcuts are intended to show the 

 position of those parts of bivalve and univalve shells 

 to which frequent allusion will be made in the descrip- 

 tions given of the several species. 



In order to ascertain which is the right and which 

 the left valve of a bivalve shell, we have only to hold 

 it in the position it would occupy if the animal were 

 crawling straight away from us, viz. with the anterior 

 or front side furthest from, and the posterior or hinder 

 side nearest to us, the beaks being uppermost ; the 

 valve on our right hand will be the right, that on our 

 left the left valve. 



LOV\iE:R MARGIN 



The first figure represents the left valve of Unio 

 tumidus, and shows the position of the anterior and 

 posterior sides^ the lower margin, ligament, and umbo. 



In the second figure the interior of both valves is 



