In fine muds such as those covering most of the bottom in the deeper parts of the Clyde Sea Area the formation of fæcal pellets is a factor of great biological importance. In these muds up to 40 per cent of the fine material is often consolidated into fæcal masses, and in extreme cases even the whole of the mud may be in the form of pellets. The significance of this process is clear. In the first place, the effective particle size of a large proportion of the mud is very greatly increased, and this in turn affects such characters as the porosity of the bottom, and the liability of the material to be transported by currents. In the second place, among selective feeders such as Syndosmya alba, which feed on the mud, there is an economy of labour, in that food which has once passed through the gut is set aside in a form in which it will be refused if again taken in by the siphons. This latter process can readily be seen in the example cited whose rejecta on examination prove to be almost entirely composed of pellets of its own and other species.