A new, short-stemmed genus with two species is described in the Stemonaceae. It has regular, 5- merous flowers, obviously the first among the Monocots. The four genera of the family are discussed and their characters contrasted. The fruit and seed of Stichoneuron are for the first time described. Attention is called for the peculiar aril in the four genera. It is concluded that the family is a natural one, in vegetative characters and seed structure, and should not be split up. A new family description is given and an artificial key to the genera. Some observations on anatomical features were checked or established, especially concerning the crystals, by Dr. P. Baas. A concise account of the palynology of the four genera is given by Dr. J. Muller. Besides the new type species, Pentastemona sumatrana. , there is one new combination for the second species, P. egregia (basionym Cryptocoryne egregia Schott).