The septal development of Oligophylloides pachythecus Rozkowska, the type species for the genus, has been studied by means of closely spaced serial sections. It has been observed that four original septa were secreted on a basal plate with two sets of adjacent- septa joining before the two resulting axial septa each join. The result is a symmetrical double 'Y', with the bases of the letters joined at the axis. Subsequently each of these four may split peripherally, producing eight septa. Next, two additional septa were added pinnately on each side on one of the open fossulae, but not within the adjacent enclosed loculae. The specimens studied have a total of only 10 septa each. The resulting septal pattern, which produces a bilateral symmetry with open fossulae in the plane of symmetry, is at marked variance with the previously assumed septal insertion plans described from Lower Carboniferous heterocorals.