Parts of a shell

(Original Forum Question : GOOD DAY COLLECTOR. I JUST WONDER WHY SHELL HAS IT'S OWN PARTS. I HAVE HD SOME WORDS WHICH I CERTAINLY DON'T UNDERSTAND. IS SOMEONE THERE WILLING TO GIVE THE MEANING OF THE FF. WORDS.
1. PERISTOME
2. LABRAL MARGINE
THAT'S ALL, BUT THERE MUST BE SOME OTHER WORDS WHICH I HAVE NOT HEARD OF. I KNOW THE WHAT IS A BASE AND A DORSUM, AND OFCOURSE, A LIP, BUT WHAT DO YOU CALL THE SHINY INTERIOR PART OF THE SHELL? IN THE CYPRAEAS BODY, THERE ARE SOMETIMES DARK SPOTS WHICH IS QUITE SMALL THAT IT IS ONLY VISIBLE IN VERY CLOSEUP VIEW, THEY LOOK LIKE STAINS THAT IS PRMANENT. I CALL THEM SMOKY BLEMISHES, BUT WHAT RE THEY NORMLLY CALLED? ANYONE?)

"Stoma" means "mouth", and refers to the "mouth" – or more correctly the "aperture" – of the shell. "Peri" means "around", as in "perimeter"; so "peristome" means "around the mouth", and refers to the rim or edge of the aperture, all the way around. "Labrum" means "lip", so the labral margin is the side of the aperture defined by the lip of the shell, as opposed to the "columellar margin", which is the side of the aperture bordering the columella, the central axis around which the shell is coiled. The interior of the shell, shiny in some species, rather dull in others, has no special name I know of. It is usually just designated the "interior surface".

The markings you refer to in Cypraea – are they in the shell or in the soft parts of the animal – the foot or mantle? I don't know of any specific terminology for such markings in the shell. But crustaceans and some other marine invertebrates have minute contractile pigment-containing structures called chromatophores in the soft tissues. That may be what you are referring to, if in fact you are referring to the soft parts.
(Answer by M. Paul Monfils via the Forum)