The morphology of eight species in the genus Frustulia from freshwaters in Scotland and Wales was investigated in light and scanning electron microscopy. They include Frustulia krammeri Lange-Bertalot & Metzeltin, the types of Frustulia saxonica Rabenhorst and Frustulia crassinervia (Brébisson) Lange-Bertalot & Krammer, Frustulia quadrisinuata Lange-Bertalot, Frustulia erifuga Lange-Bertalot & Krammer, Frustulia amphipleuroides (Grunow) Cleve-Euler, Frustulia vulgaris (Thwaites) De Toni, and a Frustulia species which resembles Berkella alpina (Amossé) Carter (1993).
Frustulia krammeri, F. saxonica, F. crassinervia and F. quadrisinuata have rhombic, rhombic-lanceolate, lanceolate or elliptic-lanceolate valves which differ in the projection and width of the valve ends. Frustulia erifuga and F. amphipleuroides are larger species with lanceolate valves, and frustules of F. vulgaris and B. alpina are smaller with linear or linear-elliptic valves with subrostrate ends. Berkella alpina was based on Frustulia spicula var. alpina Amossé (1972) nom. inval., a species described as Berkella linearis Ross & Sims (1978). The latter taxon is currently regarded as a synonym of Frustulia amosseana Lange-Bertalot, together with Frustulia vulgaris var. capitata Krasske, Frustulia spicula var. alpina and Frustulia spicula var. spicula partim (Rumrich et al. 2000). Frustulia vulgaris var. capitata and other illustrated specimens of F. amosseana have capitate apices and do not resemble Berkella alpina ≡ Frustulia spicula var. alpina ≡ Berkella linearis as illustrated from France, Scotland and Wales. These specimens have subrostrate valve ends and it is suggested that Berkella alpina should be transferred to Frustulia. In the population from Wales, found on subaerial bryophytes on a wall behind a waterfall, silica plates were observed covering the internal valve surface. Depressions on either side of the raphe in some specimens are erosions of the external valve surface.

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