Trithuria is a genus of small ephemeral aquatic herb that represent the only members of the family Hydatellaceae found in India, Australia, and New Zealand. All of the 13 formally characterized species of Trithuria are found in Australia, with the exception of T. inconspicua and T. konkanensis, which are found in New Zealand and India, respectively.  Until DNA sequence data and a reinterpretation of morphology proved otherwise, these plants were believed to be monocots related to the grasses (Poaceae).  They are unique in being the only plants besides two members of Triuridaceae (Lacandonia schizmatica and L. braziliana) in which the stamens are in the center of the flower while the pistils surround them; in Hydatellaceae the resulting 'flowers' may instead represent condensed inflorescences or non-flowers.

These diminutive, superficially moss-like, aquatic plants are the closest living relatives of a clade comprising two closely related water-lily families Nymphaeaceae and Cabombaceae. Together, these three families compose the order Nymphaeales in the APG III system of flowering plant classification. Trithuria (Hydatellaceae) diverged from the rest of Nymphaeales soon after Nymphaeales diverged from its sister taxon, although the crown clade evolved relatively recently, in the early Miocene (~19 Ma;). The order as a whole is the sister group of all flowering plants except Amborellales.

Trithuria exhibits a remarkable similarity to Centrolepis and species of both genera were mistaken for members of the other genus.

Taxonomy
The genus Hydatella was subsumed into Trithuria as its members are phylogenetically nested in it. The family as a whole shares the following features (morphological synapomorphies)
 lack of a vascular cambium
 lack of pericyclic sclerenchyma
 anomocytic stomata
 truncate anther connective
 boat-shaped pollen
 inner integument with two cell layers
 palisade exotesta
 seed operculum formed by cell enlargement in the inner integument
 perisperm
 hypogeal germination.

Species and distribution

 Trithuria austinensis D.D.Sokoloff - Western Australia
 Trithuria australis (Diels) D.D.Sokoloff - Western Australia
 Trithuria bibracteata Stapf ex D.A.Cooke - Western Australia
 Trithuria cookeana D.D.Sokoloff, Remizowa, T.D.Macfarl. & Rudall - Northern Territory of Australia
 Trithuria cowieana D.D.Sokoloff - Northern Territory 
 Trithuria filamentosa Rodway - Tasmania
 Trithuria fitzgeraldii D.D.Sokoloff, I.Marques, T.D.Macfarl., Rudall & S.W.Graham - Western Australia
 Trithuria inconspicua Cheeseman - North Island of New Zealand
 Trithuria konkanensis S.R.Yadav & Janarth. - Maharashtra
 Trithuria lanterna D.A.Cooke - Northern Territory, Western Australia, Queensland
 Trithuria occidentalis Benth. - Western Australia
 Trithuria polybracteata D.A.Cooke ex D.D.Sokoloff, Remizowa, T.D.Macfarl. & Rudall - Western Australia
 Trithuria submersa Hook.f. - Western Australia, South Australia, Victoria, New South Wales, Tasmania

Etymology
The generic name Trithuria is composed of two parts: the Greek words treis meaning "three", and thyris meaning "window". It references the dehiscence of the fruit. Specifically, it references the pericarp valves of some Trithuria species.

Cytology
The diploid chromosome count of Trithuria inconspicua subsp. inconspicua is 2n = c. 24. 
The diploid chromosome count of Trithuria submersa is 2n = 56. The diploid chromosome count of the tetraploid species Trithuria konkanensis 2n = 40. The diploid chromosome count of Trithuria australis is 2n = 14.

References

Nymphaeales genera
Aquatic plants
Nymphaeales