Family Amphianthidae Hertwig, 1882

Diagnosis (adapted from Rodríguez et al. 2012, modifications on bold): Enthemonae with basilar muscles and mesogleal marginal sphincter. Pedal disc broad, usually elongated in the transverse plane, often grasping. Column commonly with thick mesoglea, smooth or with small mesogleal papillae distally arranged in longitudinal rows. Cinclides absent or very rare (1–3), distally over the directives endocoels, with the external opening mounted on a papilla. Tentacles usually marginal on wide oral disc, their aboral sides sometimes thickened. Oral disc sometimes lobed. Mesenteries not divisible into macro- and micro-cnemes. At least six pairs of perfect and fertile mesenteries. Retractor muscles diffuse, weak. Longitudinal muscles of the tentacles ectodermal. Acontia highly reduced, being absent in many preserved specimens, when present with only basitrichs. Cnidom: spirocysts, basitrichs, pmastigophores B1, and holotrichs.