The literature on Selenomonas has roots dating back to the 19th century—and beyond—since the features and movements of living (then unclassified) crescent-shaped microorganisms from the human mouth were first described by Antonie van Leeuwenhoek in 1683.

[5] During more recent years the crescent-shaped organism observed in ruminant stomachs has been variously described as: As can be ascertained from the above nomenclature, the genus Selenomonas provides a fascinating history of scientific discovery, involving placement then re-placement in the classification systematics, oscillating between animal and bacterial kingdoms.

The most morphologically interesting members of the selenomonads are undoubtedly the large motile crescents found in the warm anaerobic nutrient-rich microecosystem provided by ruminant rumen, guinea-pig caecum (S. palpitans) and even pockets in the human gingiva (S. sputigena).

This flagella-associated structure observed in the large selenomonad can perhaps best be described as a "basal sac" formed by an invagination (in-folding) of the "polar organelle" region of the bacterial cell membrane in the middle of the concave side of the organism so that it lies directly behind the flagella.

It appears to be unique in prokaryotes so far examined since in other bacteria possessing polar organelles, the structure is situated beside and around the flagella insertion bases in the cell membrane, but never lying behind them in the cytoplasm as in the case of the large selenomonad.

It is already clear from ultrastructural features that the genus Selenomonas is most probably an artificial classification, bringing together possibly unrelated organisms, simply because of their common possession of crescent morphology and peculiar flagellar insertion location.