Diagnosis
Carapace usually less than 1mm long, ovate in lateral outline, strongly calcified, smooth or ornamented. Anterior rostral incisure always absent. Valves unequal: right valve usually overlaps the left around its entire margin. Between ten and eighteen adductor muscles arranged in two vertical rows. Seven pairs of appendages. Antenna biramous, with endopodite and exopodite almost equally developed, flattened and bearing stout chelate setae. Maxilla, fifth and sixth limbs all with epipodial (or branchial) plates. Sixth limb modified as clasping organs in males, reduced in females. Furca lamelliform with strong chelate setae.
Marine (with a few brackish water species); benthic, mostly living in fairly deep water.