Thäl'i-dan, n.

n. [Fr. thalide, from thalie, N. Lat. thalia, a kind of mollusks, from Lat. thalia, a sea-nymph. See supra.] (Zoöl.) One of a group of segregate, naked, acephalous molluscans, of which the genus Thalia is the type. They have a small crest or vertical fin near the posterior extremity of the back.