Treated as plural. A former division of rhizopods with polythalamous shells, which included most foraminiferans; (also polythalamia) animals of this division (collectively or individually).
Mid 19th century; earliest use found in American Journal of Science. From scientific Latin Polythalamia (C. G. EhrenbergDie Infusionsthierchen als vollkommene Organismen 136; in use earlier as adjective, and as a specific name of some annelids (LinnaeusSystema naturae (ed. 12, 1767) 1266)) from poly- + thalamium or its elements ancient Greek θάλαμος inner chamber and scientific Latin -ium; compare -ia.