The periostracum is a horny covering which overlays the exterior of the shell in many species and, like the shell, is secreted and shaped by the fleshy mantle of the animal. The periostracum (erroneously called the epidermis) may be very thin and transparent or only slightly tinted (as in some volutes, moon shells and the smaller conchs); or it may be like a thick coating of shellac which flakes off when dry (as in the Queen Conch, Strombus gigas). In a few buccinids, some frog shells (Lampusia) and the vase shells (Vasum), the periostracum may be very thick and often have clumps which simulate hairs and bristles. It is wholly absent in many groups, including the cowries, olives and marginellas. It is primarily a protective coating and prevents damage from boring sponges and water acids.
When axial and spiral sculpturing are equally prominent and cross each other at right angles, a cancellate or decussate sculpture is produced. Reticulate sculpture is similar, but the lines do not cross at right angles.
Growth lines are mentioned in many of our descriptions and these refer to the axial lines which run parallel to the edge of the apertural lip. These are irregularities in the shell, usually very small but sometimes coarse, which mark places where growth of the shell was stopped for a relatively long time. Sometimes the lip of the aperture becomes stained or slightly thickened during these brief rest periods (probably a few days apart), and, when additional growth takes place, these blemishes are left as growth lines.
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Figure 24. Various types of opercula. a, calcareous (Turbo); b, under surface of same showing the paucispiral, corneous layer to which the foot muscle is attached; c, calcareous and paucispiral (Nerita); d, paucispiral and corneous (Littorina); e, ungulate and corneous (Busycon and Vasum); f, multispiral and corneous (Livona); g, concentric and corneous (Buccinum).
The operculum is a horny or calcareous plate firmly attached to the dorsal side of the posterior end of the foot. When the head and foot are withdrawn into the shell, this “trapdoor” is the last part to be pulled in, and it thus serves as a protection against enemies and, in many species, seals the shell from either noxious fluids or the drying effects of the sun and air. When the foot is extended and used in crawling, the operculum serves as a foot-pad on which the heavy shell may rest and rub without injury to the soft foot. The operculum is present in many families of marine mollusks, and it often
