The type material of Ilionia prisca (Hisinger) is redescribed. This species shows several basic morphological characteristics typical of Recent Lucinacea, including an anteriorly expanded shell with conspicuous diagonal sulci, a hypertrophied anterior adductor muscle scar, and a nonsinuate pallial line. It is believed to have been a suspension feeder provided with a posterior exhalant siphon and capable of forming an anterior inhalant mucus tube in the sediment. Ilionia prisca lived in a habitat unfavorable for most other bivalves, deeply buried in a soft, oxygen-poor and sulphur-rich mud, in which it oriented itself obliquely against the direction of water movement. Probably it lived in symbiosis with chemoautotrophic bacteria. It is suggested that Ilionia prisca is the oldest known deep-burrowing suspension-feeding bivalve.