The genus Horsfieldia (Myristicaceae) comprises c. 66 species, and is distributed from Ceylon and NE. India through S. China, Indo-China and Malesia to the Solomon I. and N. Australia. It is absent from the Lesser Sunda I. Apart from a few widely distributed species, viz. the well-marked H. irya and H. macrocoma, most of the species are of limited distribution. About half of these occur in W. Malesia (& Indo-China) and have predominantly 3-valved perianths. The others which have predominantly 2-valved perianths, occur East of Wallace’s Line, mainly in New Guinea. This is the more peculiar because in most individual specimens (and species) the number of perianthvalves is not at all fixed: usually perianths with a deviating valve-number are found in low percentages. As regards the distributional separation there are a few more exceptions, viz. H. crassifolia (widely distributed in W. Malesia) has a 2-valved perianth, but the shape of the androecium closely links up with other West Malesian 3-valved species. The East Malesian H. sepikensis, on the other hand, has a predominantly 3-valved perianth, but its androecium links up with that of several related 2-valved species in the same area. In general the perianth character, though not strictly fixed even in the specimens, apparently has a significance connected with Wallace’s Line.