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.