Your question begs a bigger one: What IS a seashell? Encarta defines a seashell as "the empty shell of a sea creature, especially a mollusk." Sea biscuits, as you may know, are echinoderms, not mollusks, but they certainly are sea creatures. The hard part that washes up onto beaches is called a "test," which is really a skeleton more than a shell. So I suppose technically, no, it's not a seashell, but very few beachcombers would make the distinction.