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Hoiv to Know American Seashells 77
Tlie periostracu7n 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
( Vasimi) , 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 pre-
vents 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. Reticu-
late 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 dur-
ing these brief rest periods (probably a few days apart), and, when addi-
tional growth takes place, these blemishes are left as growth lines.
Figure 24. Various types of opercula. a, calcareous {Turbo); b, under surtace of
same showing the paucispiral, corneous laver to which the foot muscle is attached;
c, calcareous and paucispiral (Neritu); d, paucispiral and corneous {Littorina)\ e,
ungulate and corneous {Busy con and Vasmn); f, multispiral and corneous
(Livo?7a); g, concentric and corneous {Biiccinwn).
The operczilimt is a homy 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withdrawn into the shell, this "trapdoor" is the last part to be pulled in, and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