Belongs within: Delesseriaceae.
Hypoglossum is a genus of erect red algae with flat thallus blades that are monostromatic except for the midribs (Womersley 2003).
Characters (from Womersley 2003): Thallus usually erect, mostly 5–20 cm high, with flat (usually), elongate blades arising from the midrib of older blades, monopodially developed (sympodial in H. revolutum), monostromatic apart form the midrib; lateral veins absent; holdfast discoid, usually epilithic. Apical cells obconical, segmenting to form an axial filament of cells with 2 lateral pericentral cells followed by 2 transverse pericentral cells, the lateral cells forming second-order cell rows with all or some cells forming third-order rows, with all rows reaching the blade margin. Blade margin with or without fourth-order rows, entire or serrate, or with fimbriate uniseriate extensions. New branches arise endogenously from the axial cells. Midrib becoming corticated and forming a subterete stipe below as the blade wings are lost. Reproduction: Gametophytes dioecious. Procarps borne on transverse pericentral (supporting) cells on both sides of the blades, with 2 sterile groups and a 4-celled carpogonial branch. Carposporophyte with a basal fusion cell, much branched gonimoblast, and short chains of ovoid carposporangia maturing sequentially. Cystocarps ovoid to urceolate, sessile to stipitate, ostiolate and sometimes with a conspicuous neck; pericarp formed by erect filaments with 1–3 layers of outer cortical cells. Spermatangial sori on both sides of young blades, usually with sterile midrib and margin, with the primary cells cutting off initials each of which produce several spermatangia. Tetrasporangial sori on smaller blades, with tetrasporangia cut off from several inner second- and third-order cells, occasionally from lateral (or even transverse) pericentral cells, and often from cortical cells, the sorus containing sporangia of different ages and covered by a cortex 1–2 cells thick.
Hypoglossum Kützing 1843 W03
|--*H. woodwardii Kützing 1843 [incl. H. hypoglossoides (Stackhouse) Collins & Hervey 1917] W03
|--H. annae Wynne & de Clerck 2000 W03
|--H. armatum (Agardh) Agardh 1898 [=Delesseria armata Agardh 1894; incl. D. microdonthum Agardh 1898] W03
|--H. dendroides (Harvey) Agardh 1898 (see below for synonymy) W03
|--H. harveyanum (Agardh) Womersley & Shepley 1982 (see below for synonymy) W03
| |--H. h. var. harveyanum W03
| `--H. h. var. fimbriatum Womersley 2003 W03
|--H. heterocystideum (Agardh) Agardh 1898 (see below for synonymy) W03
|--H. protendens (Agardh) Agardh 1898 [=Delesseria protendens Agardh 1894] W03
|--H. revolutum (Harvey) Agardh 1898 (see below for synonymy) W03
`--H. tenuifolium W03
|--H. t. var. tenuifolium W03
`--H. t. var. carolinianum W03
Hypoglossum dendroides (Harvey) Agardh 1898 [=Delesseria dendroides Harvey 1855, Hydrolapatha dendroides (Harvey) Kuntze 1898, Membranoptera dendroides (Harvey) Kuntze 1891; incl. Hypoglossum dendroides var. lancifolia, Hypoglossum dendroides var. oblongifolia] W03
Hypoglossum harveyanum (Agardh) Womersley & Shepley 1982 [=Delesseria harveyana Agardh 1872; incl. D. serrulata Harvey 1958 non Harvey 1957, Hypoglossum serrulatum (Harvey) Agardh 1898] W03
Hypoglossum heterocystideum (Agardh) Agardh 1898 [=Delesseria heterocystidea Agardh 1885; incl. D. hypoglossoides Harvey 1855, Hypoglossum hypoglossoides (Harvey) Womersley & Shepley 1982 non (Stackhouse) Collins & Hervey 1917, Membranoptera hypoglossoides (Harvey) Kuntze 1891, H. marginatum Agardh 1898, Delesseria undulata Agardh 1894, H. undulatum (Agardh) Agardh 1898] W03
Hypoglossum revolutum (Harvey) Agardh 1898 [=Delesseria revoluta Harvey 1855; incl. Placophora cucullata Agardh 1892, D. denticulata Agardh 1894 nec Montagne 1849 nec Harvey 1855, Hypoglossum denticulatum Agardh 1898] W03
*Type species of generic name indicated
[W03] Womersley, H. B. S. 2003. The Marine Benthic Flora of southern Australia. Rhodophyta pt IIID. Ceramiales—Delesseriaceae, Sarcomeniaceae, Rhodomelaceae. Flora of Australia Supplementary Series 18. Australian Biological Resources Study: Canberra, and the State Herbarium of South Australia: Adelaide.