Etymology: Hypo-; refers to Hypoderma, the genus from which H. scirpinum has been segregated. -helion = swamp or marsh dwelling; refers to the habitat of the species included in this genus.

Johnston (1988a, 1988b, 1989, 1990) discussed the unsatisfactory nature of currently accepted generic limits in the Rhytismataceae, and showed that, at least for nonconiferous species, an alternative is possible. Rather than using single characters chosen a priori as important, genera may be based on sets of several characters, which in this case are mostly associated with ascomatal structure and development.

The characters used to define Hypohelion and to distinguish it from other genera in the Rhytismataceae are:
1. Subcuticular ascomata.
2. The upper wall of unopened ascomata is homogeneous in structure, comprising dark brown, thick-walled, angular cells.
3. In opened ascomata the upper wall is more or less uniform in thickness.
4. The edge of the ascomatal opening is not lined with differentiated cells.
5. The ascomata lack a darkened lower wall.
6. The paraphyses are swollen at the apex.

Hypoderma, in the sense accepted by Johnston (1990), possesses only the first of the characters listed above, and the upper wall of the ascomata has thickened labia and a characteristic internal differentiation. Powell (1974) considered that vestiges of thickened labia, in the form of a medial keel, could be seen in the upper wall of the ascomata in many collections of H. scirpinum. My observations showed that although a keel-like ridge is often seen on unopened ascomata, this is due to a folding, rather than a thickening, of the ascomatal wall. It cannot be compared with the thickened labia characteristic of Hypoderma rubi and other species of Hypoderma.

Hypohelion scirpinum and H. parvum are morphologically and anatomically similar, and are also similar with respect to host preference. Both are found on Cyperaceae growing in swampy habitats. The same kind of relationship between ascomatal structure and host substrate was demonstrated for a group of Lophodermium, Lophomerum, and Meloderma spp. found on the closely related families Epacridaceae, Ericaceae, and Pyrolaceae (Johnston 1988b). This correlation between morphological and biological characters suggests that the character set used in this paper, and by Johnston (1990) for Hypoderma, may define rhytismataceous genera in a way which reflects natural relationships amongst these fungi.