1. 3 species of Cymodocea and 2 of Diplanthera are found in Japan. 2. The creeping shoot of Cymodocea isoetifolia and C. rotundata has small differentiated leaves on the rhizome. 3. Cymodocea in Japan, though dioecious, are destitute of either the male flower or both sexes. 4. In Enhalus, the first leaf on the lateral branches grows sessile with the bract. 5. In Thalassia the rhizome when branching bears the axillant leaf, whereas the flowering shoot is destitute of it. 6. Halophila in Japan is considered to belong to one species, viz. Halophila ovalis, which may be divided however into three forms by their leaf form. They have only pistillate flower.