Bigelow & Alexander (2000) recorded a stalked diatom growing on the cirri of the barnacle Tetraclita squamosa. Further study of this diatom revealed a suite of characters unknown amongst araphid diatom genera. We propose a new, at present monotypic genus, Licmosoma. Comparison with data in the literature clearly suggests a position in the Licmophorales. Later, a second form of Licmosoma was found on another barnacle species Chthamalus malayensis and this also is discussed briefly.

Samples from a hard surface in a flow of water which had no accumulation of sediment or contamination from other communities living in lotic habitats yielded a distinctive river epilithic flora. The relationship of this discrete flora to that of others from diverse rivers raises interesting questions regarding the effect of water flow and contamination of communities.

Pliocaenicus taxa living or in very recent sediments from upland lakes around Lake Baikal are examined in relation to those found in Pleistocene and Miocene deposits in Kamchatcka and elswehere. The validity of the genus Pliocaenicus is substantiated and subdivision of P. costatus sensu lato is examined in some detail. Re-examination of material from Skabitchevsky's type locality and an LM and SEM study of conspecific and closely related taxa has been undertaken. Skabichevsky's type material is unavailable and two morphotypes have been described in the literature under this epithet. The taxonomy of P. costatus var. sibiricus (Skabitch.) Round & Hakansson is now clarified and a new variety, P. costatus var. leprindus, is established. Comparison with fossil material shows that P. costatus taxa are very stable over time with morphologies differing only in detail of valve central area, areolar density, valve central area texture (colliculate or smooth), spacing of marginal costae, and number of central area f...

Problems associated with the contamination of distinct diatom communities by live or dead frustules (valves) from adjacent communities are discussed. The importance of the recognition and discrete sampling of the numerous microhabitats in both freshwater and marine systems is stressed. Special comments are made concerning the complexity of the epilithic flora in rivers and the "metaphyton" associated particularly with the epiphyton.

A new genus (Gijfenia) is described from a re-investigation of Nitzschia cocconeiformis Grun. found in material deposited in the Natural History Museum (London) by Giffen. It is an extremely robust diatom with chambering of the valve in the "pinnularioid manner". A light and scanning electron microscope study of its morphology is provided together with comment on its classification. Gijfenia is compared with some other species in the literature. In the same material Nitzschia (Tryblionella) lanceola Grun. was found and in SEM the "spines" reported in the literature were discovered to be spathulate.

A new monoraphid genus (Pogoneis) is described from a sample of epiphyton growing on the red alga Sarconema filiforme. Its relationship to other achnanthoid species was studied and two of these also proved to require allocation to new genera. Achnanthes hungarica was therefore transferred to Lemnicola nov. gen. and the marine taxon A. taeniata to Pauliella taeniata nov. gen.

The nomenclature of the taxon first recorded as Fragilaria (virescens) v. exigua by Grunow in Cleve & Moeller (1878) is examined with regard to the current botanical name, F. exiguiformis Lange-Bertalot. The distribution of this taxon in relation to pH is described from surface sediment samples collected from more than 200 soft-water lakes in both NW Europe and Antarctica. The optimum pH requirement is shown to be different in the two regions with a lower optimum (pH 5.7) for the European data set. One reason for this difference is the lack of moderately acid sites in the Antarctic data set. Another is that SEM investigation of F. exiguiformis from several localities shows that at least two taxa, characterized inter alia by presence or absence of spines, are present within this species. Lack of a rimoportula indicates that taxa in the F. exiguiformis complex should be transferred elsewhere. Also, the recognition of a non-spinose taxon within F. exiguiformis indicates that a new species description is required. Consequently and because the species complex cannot be incorporated satisfactorily into existing genera, a new genus, Stauroforma, and a new species, S. inermis are described. The species epithet is retained for the spinose taxon which becomes S. exiguiformis. Only S. inermis occurs in the Antarctic samples but both species frequently co-occur elsewhere. These two species have different ecological optima but since they cannot be easily separated by the light microscope using bright-field or phase contrast optics, their relative distributions and ecological preferences in NW European lakes remain uncertain.