The method described below was evolved as a substitute for counting, which is unsatisfactory for epiphytic marine diatoms. It depends upon the estimation of the amount of diatom-silica associated with unit dry weight of the sub-stratum. The sea-weed samples are first freed from sand particles and are then treated with strong nitric acid in the usual way. In order to convert the insoluble silica of the diatoms into a soluble form suitable for chemical estimation, the material is fused in a platinum crucible with a mixture of K2CO3 and Na2CO3 and the fused mass is dissolved in (silica-free) distilled water and filtered. The amount of silica in the filtrate is determined colorimetrically according to the method of Dienért and Wandenbulcke (cf. Atkins, 1926). The method is simple in practice and saves considerable time and labour. The full procedure is illustrated by reference to a study of the periodicity of Grammatophora marina (Lyngb.) Kütz. at Swanage, Dorset.

This species (PL. I) is found all the year round on Cladophora rupestris in a pit in one of the reefs at Peverel Point, Swanage. In summer it is the only epiphytic diatom on the Cladophora, while in spring and autumn small numbers of other epiphytes (Cocconeis, Opephora, Rhoicosphenia) are present. In order to obtain a fairly representative picture of the abundance of Grammatophora at different times, samples of the Cladophora were collected monthly from a definite area in the pit, each including 3–4 cm. of the apices of five to ten Cladophora tufts cut at random.