The right margin of the mouth (the left, in sinistral shells) is termed the _outer lip_ or _labrum_, the left margin the _inner lip_, _labium_, or _columella lip_.[348] 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 extraordinary size (chap. xiv.). As growth proceeds, these marginal teeth and ribs are either dissolved and disappear, or are permanently incorporated, in the shape of _varices_, with the framework of the shell. Some shells, _e.g._ _Natica_, _Turritella_, _Actaeon_, have 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 _Conus_, or excavated, as in _Sigaretus_, _Struthiolaria_, and _Bulla_. Frequently it is continued by part of the body whorl, as in _Ficula_, _Dolium_, and _Fasciolaria_. [Illustration: FIG. 176.--_Solarium perspectivum_ Lam., from the under side.] [Illustration: FIG. 177.--Section of _Turbinella pyrum_ L., showing the plicae on the columella and the growth of successive whorls.] The folds or plaits on the columella, which are often characteristic of the genus or even family (_e.g._ Fasciolariidae, Mitridae, Turbinellidae) are not merely external, but continue down the whole spire (see Fig. 177, which also shows how successive fresh growths have thickened the columella). The whorls may be wound in a spiral, which is either hollow, as in _Solarium_, or quite compact, as in _Oliva_, _Terebra_, _Cypraea_, with every possible intermediate grade. This concavity, which varies in depth and width, is known as the _umbilicus_, and shells are accordingly spoken of as _deeply_ (_e.g._ most Trochidae and Naticidae), _narrowly_ (_e.g._ _Lacuna_, _Littorina_), or _widely_ (_e.g._ _Solarium_) _umbilicated_. When the spiral is quite flat, as in _Planorbis_ and some _Helix_, the umbilicus vanishes entirely. Shells in which the whorls are so compactly coiled on an ascending spiral that there is no umbilicus, are termed _imperforate_. [Illustration: FIG. 178.--The slit in =A=, _Hemitoma_, =B=, _Emarginula_, =C=, _Macroschisma_, =D=, _Craniopsis_, =E=, _Puncturella_, =F=, _Fissurella_.]