Starfish -- I know, not a real seashell, but you see them so often dried in shell shops along the coast, I thought I would house them here for now.
Starfish or sea stars are echinoderms About 1,500 living species of starfish occur on the seabed in all the world's oceans, from the tropics to subzero polar waters. They typically have a central disc and five arms, though some species have more than this. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates.
Starfish are actually, not fish... now they are properly called sea-stars. They are echinoderms.
whimsicalraindropcottage: (via “Starfish” Fine Art Print by Leon Heyns | RedBubble)