[Amphipoda] (Abbr. MX) in Gammaridea this pair of appendages is small, each bearing an inner lobe (=plate) (sometimes with setae), an outer lobe with spines, and attached to the latter, a palp (reduced or absent in some cases) comprising 1 or 2 articles. In Hyperiidea Physosomata (but not in the Physocephalata), the maxilla I has an inner lobe, and the palp, when present, is formed by a single article. In some Physocephalata (Lycaeidae, Oxycephalidae) the maxilla I can be rudimentary or absent.

Alternative form for Maxilla I : MX I (abbrev.).