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 



