2Q INTRODUCTION. 



Apex — The summit, tip, or highest part of the spire 

 Plate II. fig. 7, 8,9, A A A. 



Base is the opposite extremity from the apex or tip of 

 the spire. In shells with a beak or rostrum, it implies the 

 tip of such beak; Plate II. fig. 11, i^. In shells without a 

 beak it is understood to be the lower part, as before-men- 

 tioned, opposite the apex; Plate II. fig. 8 and 9, B B. In 

 the Patella and some others, the base of the shell is that 

 part on which it rests when it is laid on its mouth. In the 

 Dentalium and Teredo it is the wider end. 



Body of the shell is the first or lower whorl of the spire, 

 in which the aperture is situated, and is in general longer 

 than the remaining whorls. Plate II. fig. 10 and 12, FF; 

 and Plate III. fig. 7 and 9, FF. 



Front of the shell, is that side where the aperture is 

 situated. Plate II. fig. 7, /; and Plate III. fig. 8, /. 



Back is the opposite side to that in which the aperture 

 is placed or turned directly from the observer. Plate III 

 fig. 5 and 10, G G. 



The venter, or belly, is the most prominent part of the 

 lower whorl or body, generally situated in the vicinity of 

 the lip over the aperture ; and formed by the convexity of 

 the aperture. It is in general only made use of in describ- 

 ing shells, whose body is large in proportion to the size of 

 the spire. Plate III. fig. 5 and \0, H H. 



Sides. — The extreme edges of the shell, when viewed 

 either in front or from the back. Plate II. fig. 11, 

 KKKKKKKK. Right side is, when the shell is viewed 

 in front, that side next the observer's left hand. Left side, 

 the side with the aperture in it.* 



Ap.rture, or mouth, is that part of the lower whorl or 

 body, by which the animal protrudes itself. It affords the 

 principal generic distinctions of Univalve shells, and differs 

 very much in shape; some apertures being rounded, others 

 semilunar, angular, &c. Plate II. fig. 9 and 11, C C ; and 



* Bu* whjii the shell is placed in lis natural position, with the mouth 

 below, the right side is what is here called the left. 



