A revision of the genus in its entire range of distribution is presented. Out of more than 50 published specific names, 9 species (with 13 subspecies or varieties) are recognized, in addition to 4 new species and one new subspecies. The two subgenera Pentamastixia and Tetramastixia of Wangerin (1910) are shown to be artificial. Two new subgenera, Manglesia Matthew (2 species) and Mastixia (II species) are established. The former is separated on the basis, among others, of stamens (8), calyx (subtruncate), inflorescence branches (4- angular), and fruits (with swollen septum). The last character is expected to be of value in identifying fossil fruits. This subgenus has a disjunct distribution, broken up into two small areas, one continental, the other W. Malesian. It is possible that the two species are relics. Within the subgenus Mastixia, the two series Oppositae and Alternae are proposed, based on the arrangement of the first order branches of the inflorescence. This character has a two-fold merit: 1) that it divides the constituent taxa into 2 clear groups, almost without exception; 2) that this is an obvious character verifiable in flowering and fruiting materials of all ages. On the basis of this character, the constituent taxa show a clear pattern of distribution, with the eastern part (New Guinea and the Solomon Islands) exclusively occupied by the Oppositae and the western part (Ceylon and the Asian Continent except its Malayan extremity) entirely occupied by the Alternae, with western and central Malesia being occupied by both. It is claimed that these two series reflect two trends in evolution in the subgenus, as confirmed by the distinct pattern of distribution. From the maximum density of occurrence of members of both series, and the richest development of forms in N. Borneo and the Malay Peninsula, it is suggested that the subgenus radiated from this centre. Three other notable trends are: 1) M. rostrata has its ssp. rostrata in Java and the Lesser Sunda Islands with (sub) opposite phyllotaxis while the other ssp. caudatifolia in Borneo has alternate phyllotaxis; 2) among the Oppositae, its eastern members (New Guinea and the Solomon Islands) have 5-merous flowers while the western counterparts have 4-merous flowers; 3) the tendency to fluctuation of number of sepals, petals, and stamens, frequent in the subgenus, reaches its peak in the New Guinea—Solomon Islands area. A section devoted to ‘Notes on characters’ from analysis of numerous specimens is given as a record of information newly obtained. An appreciation of relative value of characters was reached through this search. Among the more diagnostic features in the genus (in the order of usefulness as elaborated in the keys) are: 1) phyllotaxis; 2) number of sepals, petals, and stamens; 3) length/width ratio of sepals; 4) shape and length of bracts; and 5) shape and size of fruits.