The streams of this area in West Wales arise in upland bogs, have a consistent rapidity of flow, and have little dissolved material in their waters. They are classified as "moorland waters of the high plateau" and "waters of the lower coastal area." Within each biosynoecium the animal associations are described as follows: (1) The surface-film association, (2) the free swimming association of the middle waters, (3) the limnophilous association (bottom fauna), (4) the phy-tophilous association, determined by plants, (5) the lithophilous association (stone associating animals). Comparison of lists of species shows little variation in type between highland and lowland reaches.