FIG. 182.--Various forms of opercula.] As regards shape and formation, the operculum has usually a more or less well-marked nucleus which may be central (_e.g._ _Livona_), sub-central (_Ampullaria_), lateral (_Purpura_), or terminal (_Pyrula_). As a rule, both the inner and outer surfaces are fairly flat, but in _Torinia_, _Cyathopoma_, and _Pterocyclus_ the outer surface is elevated and conically spiral, in some _Turbo_ (_e.g._ _Sarmaticus_) it is covered with raised tubercles resembling coral, while in others (_e.g._ _Callopoma_) it is scored with a deep trench. _Aulopoma_, a land genus peculiar to Ceylon, has a paucispiral operculum with hollow whorls, deceptively like a _Planorbis_; it fits over the aperture instead of into it. In _Livona_ and most Trochidae the operculum is cartilaginous and multispiral. In _Strombus_ it is narrow, curved, and often serrated like a leaf on one of the edges; in _Conus_ it is narrowly oblong and rather featureless; in _Littorina_, paucispiral and always cartilaginous. In many cases (_e.g._ _Paludina_) there is no true spiral form, but the striae are concentric to a nearly central nucleus, and thus give the appearance of a spiral. The evolution of the operculum in _Navicella_ from _Nerita_ has already been illustrated (p. 10). _Neritopsis_ has a very remarkable operculum, the striated appendage of which locks behind the columella of the shell, like the tooth in the opercula of the Neritidae. [Illustration: Pyrula Purpura Littorina Aulopoma Torinia × 3/2 × 3 Neritopsis Strombus Conus × 2 FIG. 183.--Various forms of opercula.] =Terms employed to denote various parts of the Bivalve Shell.=--The _umbo_, or _beak_, is the apex of the hollow cone, of which each valve may be regarded as consisting. This apex is usually more or less twisted: it is markedly spiral in _Isocardia_, _Diceras_, some _Chama_, and especially _Requienia_, while in _Pecten_, _Lepton_, and others the spiral is altogether absent. As a rule the umbones point _forward_, _i.e._ towards the anterior end of the shell. In _Donax_, _Nucula_, and _Trigonia_, however, they point backward. The umbones are generally more or less approximated, but in _Arca_ they are widely separated. An _equilateral_ shell is one in which the umbones are more or less central with regard to its anterior and posterior portion, while in an _inequilateral_ shell the umbones are much nearer one end than the other. On the other hand, _equivalve_ and _inequivalve_ are terms used to express the relation of the two valves to one another as a whole. Thus nearly all bivalve shells are more or less inequilateral, but a comparatively small proportion are inequivalve. The _dorsal margin_ is adjacent to, the _ventral margin_ opposite to, the umbones. The _anterior_ and _posterior margins_ are respectively the front and hinder edges of the shell.