Hemidiscus Wallich, 1860

Holotype species: Hemidiscus cuneiformis Wallich

Currently accepted name for the type species: Actinocyclus cuneiformis (Wallich) F.Gómez, Lu Wang & Senjie Lin

Original publication and holotype designation: Wallich, G.C. (1860). On the siliceous organisms found in the digestive cavities of the Salpae, and their relation to the flint nodules of the Chalk Formation. Transactions of the Microscopical Society of London, New Series 8: 36-55, pl. II.

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Description: Cells shaped like orange segments, the valve faces lying at an acute angle to each other. Plastids small, discoid. The genus is sometimes recorded in lists of plankton under the incorrect name Euodia. A marine planktonic genus of widespread distribution, particularly in the warmer oceans but carried in warm currents to temperate seas. Valves cuneiform, often with a central swelling on the ventral side. Valve face flat, mantle shallow and distinguished by its smaller areolae. Valve areolae radiating from an indistinct central annulus; pattern somewhat irregularly sectored (fasciculate). Areolae loculae, opening externally through cribra sunk below the valve surface; internally with round, rimmed foramina. Breaks in the valve show the internal expansion of the areolae and a slight bullulate structure within the walls. Rimoportulae confined to a row along the ventral margin. Internally they are shortly stalked auricular structures, the slits being more-or-less parallel to the valve margin. The external openings of the rimoportulae are expanded parallel to the plain valve margin. A single pseudonodulus occurs, slightly inward of the line to rimoportulae. The pseudonodulus seems to open by means of a simple aperture. Copulae wider on dorsal than ventral side, split, and ligulate, the splits occurring a little way from the apices. Substructure of the copulae not known but they appear to be solid.

The most recent alteration to this page was made on 2021-10-25 by M.D. Guiry.

Taxonomic status: This name is currently regarded as a synonym of Actinocyclus.

Gender: This genus name is currently treated as masculine.

Most recent taxonomic treatment adopted: Gómez, F., Wang, L., Hernández-Becerril, D.U., Lisunova, Y.O., Lopes, R.M. & Lin, S. (2017). Molecular phylogeny suggests transfer of Hemidiscus into Actinocyclus (Coscinodiscales, Coscinodiscophyceae). Diatom Research 32(1): 21-28.

Comments: The structure of Hemidiscus is very similar to that of Actinocyclus and some workers include it within that genus but we propose to keep it separate until further studies can be made. See also Simonsen (1972), who discusses this and the unrelated genus Palmeria. The ventral row of spinulae referred to in the literature are almost certainly the rimoportulae and not external ornamentation of the valve.

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Some of the descriptions included in AlgaeBase were originally from the unpublished Encyclopedia of Algal Genera, organised in the 1990s by Dr Bruce Parker on behalf of the Phycological Society of America (PSA) and intended to be published in CD format. These AlgaeBase descriptions are now being continually updated, and each current contributor is identified above. The PSA and AlgaeBase warmly acknowledge the generosity of all past and present contributors and particularly the work of Dr Parker.

Descriptions of chrysophyte genera were subsequently published in J. Kristiansen & H.R. Preisig (eds.). 2001. Encyclopedia of Chrysophyte Genera. Bibliotheca Phycologica 110: 1-260.

Linking to this page: https://www.algaebase.org/search/genus/detail/?genus_id=44406

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Cite this record as:
M.D. Guiry in Guiry, M.D. & Guiry, G.M. 25 October 2021. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. https://www.algaebase.org; searched on 25 June 2024

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