(paleontology) An extinct order of Cystoidea in which the thecal canals crossed the sutures at the edges of the plates, so that one-half of any canal lay in one plate and the other half on an adjoining plate.

An extinct class of Echinodermata in which the thecal canals crossed the sutures at the edges of the plates, so that one-half of any canal lay in one plate and the other half on an adjoining plate. The canals sometimes occurred in lozenge-shaped clusters, called rhombls (see illustration). The theca was ovoid and sessile in earlier forms and comprised numerous irregular plates. The ambulacral grooves were trimerous and were restricted to the brachioles. In later forms the thecal plates became fewer and larger, and were arranged in five horizontal rows, termed cycles. A stem developed aborally and the ambulacra became pentamerous, traversing part of the theca before ascending the brachioles. Rhombifera probably died out in the Devonian. See also Echinodermata.

Ordovician Pleurocystis. (Simplified after O. Jaekel)