Turbo (Sarmaticus) Turbo (Callopoma) Livona Ampullaria Natica
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.
Pyrula Purpura Littorina Aulopoma × 3 Torinia × 2 Neritopsis Strombus Conus × 3/2
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.