22 INTRODUCTORY. 



order are formed principally from the shape of the aperture, ta- 

 ken in conjunction with the general shape of the shell ; from 

 the spire being lengthened or depressed, being with or without a 

 canal, the length of the beak and its direction, together with the 

 particular form of the outer lip : the colour of shells only 

 serves as a specific distinction, and cannot in this respect, in all 

 cases, be depended upon, although in others it is an unvarying 

 test. The particular manner in which the spots are disposed, 

 frequently characterises species. 



Apex. The summit, tip or highest part of the spire. Plate 

 II. fig. 7, 8, 9. AAA. 



Base is the opposite extremity from the apex, or tin of the 

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

 such beak ; Plate II. fig. 11. B. In shells without a beak it is 

 understood to be the lower part, as before-mentioned, 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 11. fig. 10 and 12. FF. and Plate III. 

 fig. 7 and 9. FF. 



Front oi 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 pla- 

 ced or turned directly from the observer. Plate III. fig. o and 

 10. GG. 



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 describing shells, whose body 

 is large in proportion to the size of the spire. Plate III. fig. 5 

 and \Q. R H. 



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

 ill front or from the back. Plate II. tig. 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. 



Aperture, or mouth, is that part of the lower whorl or body, 



