F. E. Round

REVISION OF THE GENUS FRAGILARIA

A large number of species usually placed in Fragilaria have been examined, mainly by scanning electron microscopy. As a result Fragilaria is more narrowly circumscribed, with F.capucina as the type species. Six species are definitely placed in Fragilaria and others noted for further investigation. Five genera are separated from Fragilaria; these are Staurosira (to encompass the ‘construens’ group), Staurosirella (type species S.lapponica = F.lapponica), Pseudostaurosira (type species S. brevistriata = F. brevistriata), Punctastriata (type species P. linearis) and Neofragilaria (type species N.virescens = F.virescens). Some species other than the type have also been transferred to the new genera and others indicated as requiring further study. The characters used for their distinction are discussed, and include valve striation, linking spines, apical pore fields, rimoportulae, girdle and plastids.

FOUR NEW GENERA BASED ON ACHNANTHES (ACHNANTHIDIUM) TOGETHER WITH A RE-DEFINITION OF ACHNANTHIDIUM

The genus Achnanthidium comprises a collection of monoraphid species common in freshwaters and less so in marine habitats, though the latter are relatively uninvestigated as far as this genus is concerned. We propose that Achnanthidium be confined to the taxa around A. minutissima. Some of the remaining taxa are transferred to four new genera, Planothidium, Rossithidium, Karayevia and Kolbesia. This still leaves a large number of species to be re-allocated and these, for the moment, will have to be left as Achnanthidium sensu lato.

Persistent, vertical-migration rhythms in benthic microflora.: II. Field and Laboratory Studies On Diatoms From The Banks Of The River Avon

The vertical migration of two Euglena species and several diatom species into and out of the sediment on the banks of the River Avon has been studied under natural conditions. All species have been shown to migrate vertically upwards when exposed during daylight. Tidal flooding of the sediment is generally preceded by re-burrowing of the algae beneath the surface. Methods have been devised to follow these migrations in both the field and laboratory. Laboratory experiments show that these migrations are rhythmic, continuing under constant illumination and temperature and removed from tidal influence. The effect of three different temperatures and three different light intensities has been investigated. Transfer from low to high temperatures has been shown to reset the phase of the rhythm. The results are discussed in relation to other work and to the ‘biological clock’ hypothesis.

PERSISTENT, VERTICAL-MIGRATION RHYTHMS IN BENTHIC MICROFLORA. VI. THE TIDAL AND DIURNAL NATURE OF THE RHYTHM IN THE DIATOM HANTZSCHIA VIRGATA

1. The diatom, Hantzschia virgata, appears on the surface sands of Barnstable Harbor, Mass., during daytime low tides. Surface accumulations of this organism reach such concentrations that the sand takes on a golden-brown color. As the tide returns the cells re-burrow into the sand.2. The cells can be prevented from emerging onto the surface sands at low tide by artificially darkening the area with an opaque covering just as the tide recedes. Cells already on the surface can be made to re-burrow by similarly placing them in darkness.3. The vertical-migration rhythm will persist in the laboratory in constant illumination, constant temperature, and away from the influence of the tide for as long as eleven days. During this time the cells remain in approximate synchrony with the feral cells in nature.4. In nature, when the times of low tide approach sunset, the cells rephase their rhythm to the early morning hours of daylight. Cells collected during late afternoon low tides and returned to L:D or L:L in the ...

REVISION OF THE GENUS SYNEDRA EHRENB

A large number of species originally assigned to Synedra have been investigated mainly by scanning electron microscopy. Most are marine or brackish in distribution and structurally they differ considerably from Synedra as defined from the freshwater species and hence we have been able to recognise five new generic taxa—Catacombas, Hyalosynedra, Tabularia, Ctenophora and Neosynedra. The characters used for their distinction are discussed.

REVISION OF THE GENUS ACHNANTHES SENSU LATO. PSAMMOTHIDIUM, A NEW GENUS BASED ON A. MARGINULATUM

This paper concerns a revision of some Achnanthes subsection Achnanthidium or genus Achnanthidium species, common on sand grains in freshwaters. The taxonomy of the species required a scanning electronmicroscopy study of new and old material and examination of some type material. The SEM study has provided illustrations of 18 species, some previously unpublished. A new genus Psammothidium is split off Achnanthidium Kutz. and 25 new combinations made for species and infraspecific taxa. One species Psammothidium bristolicum Bukht. is also described from the River Spey, Scotland. Short comments are provided to assist in the identification of each taxon.

CYCLOTELLOID SPECIES FROM A DIATOMITE IN THE HARZ MOUNTAINS, GERMANY, INCLUDING PLIOCAENICUS GEN. NOV.

Three centric taxa are described from a diatomite deposit in the Harz mountains, Niedersachsen, Germany. The delimination of Cyclotella, Cyclostephanos and Stephanodiscus is discussed and a tighter generic circumscription proposed. Cyclotella radiosa cf. var. pliocaenica is explored and a new genus Pliocaenicus is erected together with two new species, P. hercynicus and P. undulatus. Cyclostephanos pygmaea, C. costatus, C. omarensis and C. pantocseki are also transferred to Pliocaenicus.

A NEW MONORAPHID DIATOM GENUS (POGONEIS) FROM BAHRAIN AND THE TRANSFER OF PREVIOUSLY DESCRIBED SPECIES A. HUNGARICA AND A. TAENIATA TO NEW GENERA

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.

TAXONOMIC STUDIES OF MARINE GOMPHONEMOID DIATOMS

ABSTRACT A comparison of freshwater and marine Gomphonema species has shown that the ecological separation is accompanied by a morphological one. They have only been assigned to the same genus because they all have heteropolar, cuneate cells, which are attached to the substratum by a mucilage pad or stalk. The marine taxa can be split into four clusters, each of which differs from freshwater Gomphonema species on many points of valve and cingulum morphology.

The distribution and classification of the problematic Fragilaria (Virescens v.) exigua Grun.Fragilaria exiguiformis (Grun.) Lange-Bertalot: a new species or a new genus?

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 requ...