Hoiv to Know American Seas hells 



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 epi- 

 dermis) 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 coat- 

 ing 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 withdrawn into the shell, this "trapdoor" is the last part to be pulled in, and 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 



