‖n. pl. [ NL. See Echinus, and -oid. ] (Zoöl.) The class Echinodermata which includes the sea urchins. They have a calcareous shell, usually more or less spheroidal or disk-shaped, composed of many united plates, and covered with movable spines. See Spatangoid, Clypeastroid. [ Written also Echinidea, and Echinoida. ] [ 1913 Webster ]

‖n. pl. [ NL. See Paleo-, and Echinoidea. ] (Zool.) An extinct order of sea urchins found in the Paleozoic rocks. They had more than twenty vertical rows of plates. Called also Palaeechini. [ Written also Palaeechinoidea. ] [ 1913 Webster ]

‖n. pl. [ NL. See Echinus, and -oid. ] (Zoöl.) The class Echinodermata which includes the sea urchins. They have a calcareous shell, usually more or less spheroidal or disk-shaped, composed of many united plates, and covered with movable spines. See Spatangoid, Clypeastroid. [ Written also Echinidea, and Echinoida. ] [ 1913 Webster ]

‖n. pl. [ NL. See Paleo-, and Echinoidea. ] (Zool.) An extinct order of sea urchins found in the Paleozoic rocks. They had more than twenty vertical rows of plates. Called also Palaeechini. [ Written also Palaeechinoidea. ] [ 1913 Webster ]