Etymology: bi- = two; vallum = wall. Refers to the early development of both upper and lower darkened walls in the ascomata.

Bivallum has all the features typical of the family Rhytismataceae (sensu Cannon and Minter 1986), and within the family is characterised by the following features:
- Ascomata elliptic to ovate in outline, with a single, longitudinal opening split.
- Ascomata subcuticular or intraepidermal.
- The ascomata develop a darkened lower wall at an early stage in ascomatal development, before the hymenium is differentiated, and before a darkened upper wall has developed.
- A layer of thin-walled, cylindrical cells develops across the exposed face of the broken upper wall of opened ascomata, but may be poorly developed and/or lost soon after the ascomata open. This layer is referred to as the 'lip cells' in the discussion.
- Paraphyses persistent, undifferentiated or swollen at apex.
- Asci saccate to subsaccate, developing sequentially.
- Ascospores variable in shape, surrounded by a thick gelatinous sheath.
- Anamorph, when present, comprises small, round, immersed conidiomata, the lower wall lined with a palisade-like layer of solitary conidiogenous cells which proliferate either percurrently or sympodially. Conidia are narrow-cylindrical, nonseptate, hyaline.
- Occurs on southern hemisphere Cupressaceae or Podocarpaceae, as either parasites or saprophytes.

Ascomata in surface view 0.4-0.6 x 0.3-0.4 mm, oblong-elliptic to more or less ovate in outline. In unopened ascomata, walls pale translucent yellowish grey with dark grey line marking edge of ascomata. Immediately prior to the ascomata opening a very pale line often forms along the future line of opening. In opened ascomata the wall is grey to pale grey with a narrow, dark line marking the outside edge of the ascomata. The single, longitudinal opening split is lined with a well differentiated, broad, white to yellowish zone.

Ascomata subcuticular. In vertical section at an early stage of ascomatal development, when paraphyses have started to form but asci are not yet evident (Fig. 1A), the lower wall consists of 3-4 rows of angular cells, the outermost 1-2 rows thick-walled and dark brown, the inner rows hyaline and thin-walled. At this stage the upper wall is up to 30 µm wide, mostly of thin-walled, hyaline, angular cells, although a few cells toward the outside of the wall are becoming slightly darkened. The inner edge of the wall is lined with short-cylindrical cells more or less free at their tips. In ascomata where asci are starting to develop (Fig. 1B), the upper wall is up to 60 µm thick near the centre of the ascomata but becomes suddenly narrower toward the edges. The inner part of the wide part of the wall is a 10-15 µm wide layer of narrow-cylindrical, irregularly branching and somewhat tangled hyphae. The outer part of this central part of the wall is of pale brown, thin-walled, angular cells. The narrower parts of the wall are of darker brown, slightly thick-walled, angular cells. The opening split forms through the central, paler part of the wall. The split starts from the outside of the wall, and as the wall breaks a palisade-like layer of hyaline, cylindrical, unbranched cells develops across the exposed face of the breaking wall. The wall often starts to break open, and the layer of cylindrical cells begins to form along the break, prior to the covering host cuticle breaking. In opened ascomata (Fig. 1 C), the upper wall is up to 80 µm thick near the opening and there consists of pale brown, thin-walled cells; the rest of the wall is made of brown, slightly thick-walled, angular cells. The layer of tangled, hyaline cells which was present toward the inside of the upper wall in unopened ascomata is lost after the ascomata open. The exposed face of the broken upper wall is lined with a 20-30 µm wide, persistent, palisade-like layer of hyaline, cylindrical cells. The lower wall now consists of 3-4 layers of dark brown, thick-walled cells.

Paraphyses 1-2 µm diam., more or less undifferentiated at the apex, extending 5-10 µm beyond asci. Asci 180-240 x 17-22 µm, saccate with broadly rounded apex, wall undifferentiated at apex, 4-spored at maturity, with 4 spores aborting. Ascospores 37-44 x 7-11 µm, oblong-elliptic with broadly rounded ends, often slightly constricted near centre, nonseptate, surrounded by a 5-8 µm wide gelatinous sheath.

Structures resembling conidiomata of Rhytismataceae appear to be associated with ascomata in some collections. Conidiomata round in outline, 0-2 mm diam., immersed. Conidiogenous cells and conidia not seen.

Etymology: refers to the geographic distribution of this species.

Macroscopically and in vertical section B. zelandicum is very similar to the Tasmanian B. microstrobi.

Coccomyces cupressinum Johnston also occurs on D. cupressinum in New Zealand. C. cupressinum can be distinguished by its cylindrical asci and filiform ascospores, and ascomata which usually open by several radiate splits and which are darkened along the edge of the opening split (Johnston 1986).