1. Growth in Myriodesma results from the activity of a three-sided apical cell. Three tissues are formed: the meristoderm, cortex, and medulla. Surface growth results from the meristematic activity of the meristoderm cells. 2. The conceptacles are bisexual. Tufts of paraphyses occur at the center of the floor of the conceptacles. Each oogonium produces a single egg. Antheridia are stalked and may be borne singly on the floor of the conceptacles or in tufts on branched hairs. 3. Segmentation of the zygote starts within 18 hours after fertilization. Four primary rhizoids are formed on the embryo. 4. The affinities of Myriodesma are thus clearly with Cystoseira, Halidrys, Scaberia, and Cystophyllum. It has some features in common with the Cystoseiraceae and some with the Sargassaceae. On the whole the balance appears to be with the Cystoseiraceae on grounds of morphology, shape of apical cell, pattern of conceptacle development, the segmentation of the embryo, and the formation of four primary rhizoids.