Codium amphibium was described by Harvey (1844) on the basis of material sent to him by David Moore, who in turn had received it from the collector, William M'Calla. This material was said to grow 'on turfy banks at extreme high-water mark, near Roundstone, county Galway', and was described as comprising numerous erect, small (to 1/4 in. high), simple, cylindrical or subclavate, obtuse fronds arising from a mass of entangled, divaricately branched filaments densely aggregated into widely spreading patches. Harvey's illustration clearly shows the habit, but his drawing of utricles is neither diagnostic nor accurate [as judged from our study of the type material]. Harvey made particular note of the habitat, which he inferred would place the plant 'beyond the reach of the ordinary sea-level'. In a later work, Harvey (1846, pi. 35 B) illustrated this species with similar drawings and added nothing to the description except to extend the maximum height of the fronds to 'nearly half an inch'. Still later, Harvey (1849, p. 194) increased the maximum height of the fronds to an inch, modified ' simple' to ' usually simple, rarely emarginate or forked', added the locality 'at the head of Birtirbui Bay, Galway' and changed the habitat as follows: 'Turf banks, near high-water mark, but washed by every tide...' Kiitzing (1849, p. 502) furnished a description similar to the original one, based on an Irish specimen sent to him by Berkeley, probably topotype material. In his illustration of this material, Kiitzing (1856, pi. 96, fig. a) snowed an important character of the utricles: a tendency towards pointed apices.