echi·no·cys·toi·da \“+sə̇ˈstȯidə\noun plural Usage: capitalized Etymology: New Latin, irregular from Echinocystites, fossil genus of Echinodermata (from echin- + -cystis + -ites -ite) + -oida : an order of Silurian fossil echinoids having small irregular spheroidal or flattened tests