Ceramium byssoideum Harvey (1853: 218) was described from specimens growing on gorgonian corals at Key West, Florida. Mazoyer (1938b: 323) merged this species with Ceramium gracillimum ``Griff. et Harv.'' on the basis of specimens sent by Harvey to Montagne and housed at Paris (PC). Erroneously considering the type of C. gracillimum to be British, she designated the warm-water variety (including Mediterranean material) as var. byssoideum (Harvey) Mazoyer. In fact, C. gracillimum was not described by Griffiths & Harvey (in Harvey, 1848 [1846--1851]: pl. CCVI) as a new species, but was a transfer to Ceramium of Hormoceras gracillimum Kützing (1842: 733; type locality: Trieste, Italy). According to Womersley (1978: 236), the type of H. gracillimum does not agree with the usual concept of C. gracillimum, but is probably related to C. diaphanum (Lightfoot) Roth (q.v.).
Hormoceras gracillimum was first transferred to Ceramium by Zanardini (1847: 223), a year previous to the same action by Griffiths & Harvey. The resulting binomial is untenable because it is a later homonym of Ceramium gracillimum C. Agardh (1824: 140; type locality: Atlantic France). I have not found any subsequent mention of the latter species, but from the description and placement within the genus [``Tribus Cer. Rosei. (Rami pinnulati, pinnis alternis).''] it may be deduced that it is not referable to Ceramium. It may be the same entity described later by C. Agardh (1828: 168) as Callithamnion gracillimum, based on a specimen from Atlantic France sent to Agardh by Grateloup under the name Mertensia delicatissima. This species was referred by Feldmann-Mazoyer (1941: 390) to Compsothamnion thuyoides (J.E. Smith) Nägeli (see Maggs & Hommersand, 1993: 164).
A name similar to Ceramium byssoideum that must be considered is C. byssoides (Goodenough & Woodward) Ducluzeau (1805: 66) (Fucus byssoides Goodenough & Woodward, 1797: 229--231; lectotype locality: Christchurch, Hampshire, England fide Maggs & Hommersand, 1993: 302), referable to Brongniartella byssoides (Goodenough & Woodward) Schmitz (see Maggs & Hommersand, l.c.). Two distinct adjectival epithets are involved: byssoideus, a Group A adjective (Stearn, 1966: 396, 92) with the endings -us, -a, and -um in the nominative singular; and byssoides, which remains invariable for all three genders in the nominative singular (Stearn, 1966: 97, the paradigm being bryoides). Despite this etymological distinction, these epithets are to be treated as if they were identical in accordance with the principle of parahomonymy set forth in Art. 53.3, viz., that names so similar that they are likely to be confused are to be treated as homonyms. The governing paradigm is the example of pteroides and pteroideus (Ex. 8). Since Ceramium byssoideum Harvey must be treated as a later homonym, it is not available as a basionym. The intended combination C. gracillimum var. byssoideum is treated as a nomen novum and accredited solely to Mazoyer in accordance with Art. 58.3.
Womersley (1978: 234) placed Ceramium byssoideum Harvey in the synonymy of C. flaccidum (Kützing) Ardissone (1871: 40) (Hormoceras flaccidum Kützing, 1862: 21, pl. 69: figs. a--d). Ceramium flaccidum Harvey is a manuscript name cited by Harvey (1848 [1846--1851]: pl. CCVI) as a synonym of C. gracillimum. Harvey had given this manuscript name to a collection that he had made in 1844 at Kilkee, County Clare, Ireland. A specimen from this collection was sent by Harvey to Kützing, who published it as Hormoceras flaccidum.