Hydromedusae with umbrella margin entire, not deeply-lobed; with conspicuous marginal nematocyst ring; manubrium with or without gastric peduncle; with circular and radial canals, and gonads usually conFned to the latter; centripetal canals either present or absent; tentacles marginal, either solid, or solid and hollow occurring simultaneously; statocysts of ecto-endodermal origin, either free or enclosed; velum generally well-developed, exceptionally absent.
The diagnosis given above combines those provided by Kramp (1968) and Bouillon et al. (2006), and adds the case when the velum is absent, as it will be shown below for the genus Glaciambulata gen. nov.
The taxonomic status of some of the families included in Trachymedusae is unsettled yet. For example, Collins et al. (2008) have shown that the family Geryoniidae Eschscholtz, 1829 is derived from within another order, Limnomedusae Kramp, 1938, and the family Halicreatidae Fewkes, 1886 belongs to a sister group of a clade containing the orders Narcomedusae Haeckel, 1879 and Actinulida Swedmark & Teissier, 1959, as well as the trachymedusan family Rhopalonematidae Russell, 1953. In addition, according to the same source, some characters of the family Petasidae Haeckel, 1879, especially the presence of 4 radial canals instead of generally 8, common to Rhopalonematidae, suggest that it may belong elsewhere.