206 MANUAL OF THE MOLLUSCA. 



In the adult condition of some shells the apex is alwaj's truncated 

 (or decollated) as in cylindrella and Bulimus decoUatus ; in others 

 it is only truncated when the animals have lived in acidulous 

 "waters (e.g. cerithidea and pirena), and specimens may be 

 obtained from more favourable situations with the points 

 perfect. 



The line of channel formed by the junction of the whorls is 

 termed the suture. 



The last turn of the shell, or hody-whorl, is usually very 

 capacious ; in the females of some species the whorls enlarge 

 more rapidly than in the males (e.g. Buccinum undatum). The 

 "base" of the shell is the opposite end to the apex, and is 

 usually the front of the aperture. 



The aperture is entire in most of the vegetable feeders {holos^ 

 tomata), but notched or produced into a canal ^ in the carnivorous 

 famihes {siqhonostomata) ; this canal, or siphon, is respiratory in 

 its office, and does not necessarily indicate the nature of the 

 food. Sometimes there is a posterior channel or canal, which 

 is excurrent, or anal, in its function (e.g. stromhidce and ovulum 

 volva) ; it is represented by the slit in scissurella, the tube of 

 typhis^ the perforation in fissurella, and the series of holes in 

 haliotis. 



The margin of the aperture is termed the peristome ; sometimes 

 it is continuous {cydostoma), or becomes continuous in the adult 

 (carocolla) ; very frequently it is " interrupted," the left side of 

 the aperture being formed only by the body-whorl. The right 

 Bide of the aperture is formed by the outer lip {lahrum), the 

 left side by the inner or columellar lip [labium), or partly 

 by the body- whorl (termed the "wall of the aperture," by 

 Pfeiffer). 



The outer lip is usually thin and sharp in immature shells, and 

 in some adults (e.g. helicella and hulimidus) ; but more frequently 

 it is thickened ; or reflected ; or curled inwards {inflected), as in 

 cyproea ; or expanded, as in pteroceras ; or fringed with spines, as 

 in murex. When these fringes or expansions of the outer lip 

 are formed periodically, they are termed varices. 



Lines of colour, or sculpture, running from the apex to the 

 aperture are spiral or longitudinal, and others which coincide 

 with the lines of growth are " transverse," as regards the whorls; 

 but stripes of colour extending from the apex across the whorls 

 are often described as "longitudinal" or "radiating," with 

 respect to the entire shell. 



Shells which are always concealed by the mantle are colourless, 

 like Umax and parmophonis ; and those which are covered by the 



