206 MANUAL OF THE MOLLUSCA. 
In the adult condition of some shells the apex is always truncated 
(or decollated) as in cylindrella and Bulimus decollatus ; 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 i is 
termed the suture. 
The last turn of the shell, or body-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 
families (sighonostomata) ; 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. strombide and ovulum 
volva) ; it is represented by the slit in scisswrella, 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 (cyclostoma), 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 
side of the aperture is formed by the outer lip (labrum), the 
left side by the inner or columellar lip (labiwm), 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 bulimulus) ; but more frequently 
it is thickened ; or reflected; or curled inwards (inflected), as in 
cyprea ; or expanded, as in pteroceras ; or fringed with spines, as 
in murec. 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 limaz and parmophorus ; and those which are covered by the 
