264 



PARTS OF THE UNIVALVE SHELL 



CHAP. 



is termed the outer lip or lahrum^ the left margin the inner lip^ 

 labium^ or columella Up} In young shells the outer lip is 

 usually thin and unfinished, while in the adult it is generally 

 thickened into a rib, or furnished with more or less prominent 

 teeth, or given an inward or outward curve. In some genera, 

 especially the Strombidae, the outer lip of the adult develops 

 long finger-like processes, which sometimes attain an extraor- 

 dinary size (chap. xiv.). As growth proceeds, these marginal 

 teeth and ribs are either dissolved and disappear, or are perma- 

 nently incorporated, in the shape of varices^ with the framework 

 of the shell. Some shells, e.g. Natica^ Turritella^ Actaeon^ have 



Fig. 176. — Solarium, perspectimim 

 Lam., from the underside. 



Fig. 177. — Section of Turhinella 

 pyrum L., showing the plicae 

 on the columella and the growth 

 of successive whorls. 



a permanently unfinished outer lip, even in the adult stage. 

 The columella lip varies in shape with the mouth as a whole ; 

 thus it may be straight, as in Coiius^ or excavated, as in 

 Sigaretus^ Struthiolarla^ and Bulla, Frequently it is continued 

 by part of the body whorl, as in Fieula, Dolium^ and Fasciolaria. 

 The folds or plaits on the columella, which are often charac- 

 teristic of the genus or even family (j^.g. Fasciolariidae, Mitridae, 

 Turbinellidae) are not merely external, but continue down the 



1 The columella, as distinct from the cohimella lip, is the solid pillar of shell 

 round which the whorls are coiled (Fig. 177), the lower, or anterior portion of 

 which alone is usually visible. 



