20 THE LIFE OF THE MOLLUSCA 
more often it is ‘interrupted,’ the side next the 
columella being bounded by the wall of the body 
whorl only, when the exterior portion is known as 
the “outer lip” or “labrum,” and the other side as 
the “inner,” “ columella lip,” or ‘‘ labium.”’ There 
is frequently a notch in the margin of the aperture 
near the junction of the outer lip with the body 
whorl, and another where it joins the anterior end 
of the columella; these are respectively termed the 
*‘ posterior’ and “anterior canal,” and give passage 
to the siphons of the animal. (All these terms are 
illustrated on Plate VI.) This group has been called 
‘‘siphonostomatous,” and those without any notch 
“‘ holostomatous.”’ 
In some rhipidoglossate forms (Pleurotomaria, the 
fossil Belerophon, Emarginula, etc.) and the toxo- 
glossate Pleurotoma there is a slit near the periphery 
of the labrum that gives passage to the anal fold or 
siphon. In others—Haliotis, etc.—the slit gives 
place to a series of perforations (Plate VII, 
Figs, 1-8. See also infra, pp. 94.and 95). 
The labrum is thin and sharp in most shells, and 
in some adult forms; but more frequently it is 
either thickened, or curved outwards. (reflected), or 
curled inwards (inflected), or expanded or fringed” 
with spines. When these thickenings or expansions 
occur periodically during the growth of the shell, . 
they form conspicuous transverse markings on the. 
whorl termed “ varices.” The exterior of the shell 
