UNIVALVE MOLLUSKS 



119 



~A} 



—oL 



-•^n. 



shell you notice that this aperture is on the side next 

 to your right hand. This is true of most shells, 

 which are therefore called dextral; though a few, 

 which turn the other way, as in Figure io2, are 

 known as left-handed, or sinistral shells. 



The upper part of the shell 

 is known as the spire, marked 

 sp^ of which the very top, ^, 

 is the apex, while each turn of 

 the spire is a whorl, the larg- 

 est of which, b. tc\, is the body 

 whorl. The spiral groove be- 

 tween the whorls is the suture, 

 marked s. The central post, 

 r, is the columella, while the 

 central opening, ?/, is the um- 

 bilicus. The little open tube 



marked ca is called the canal, and the outer edge ot 

 the aperture, o. /., is the outer lip. The inner lip 

 in this specimen is grown to the columella and does 

 not appear, though in some shells, as shown in Fig- 

 ure 156, it is very evident. Many shells have no 

 canal, and in most instances we judge that the ani- 

 mal which has such a shell is herbiverous, while those 

 with canals are carnivorous. 



Lines of growth run parallel to the edge of the 

 outer lip, while spiral lines run around the shell and 

 cut the lines of growth. Varices are enlarged por- 

 tions of the shell, parallel to the lines of growth, 

 while the operculum is a kind of door that the ani- 

 mal pulls up to the aperture, after he has withdrawn 

 into his shell. With these few simple definitions, 



Fig. 91 



