Hemidiscus Wallich, 1860
Holotype species: Hemidiscus cuneiformis Wallich
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.
Taxonomic status: currently recognized as a distinct genus.
Most recent taxonomic treatment adopted: Tomas, C.R., Eds (1996). Identifying marine diatoms and dinoflagellates. pp. 1-858. San Diego: Academic Press Inc.
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 20 Oct 2011 by M.D. Guiry.
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.
Numbers of names and species: There are 22 species names in the database at present, as well as 4 infraspecific names. Of the species names, 2 have been flagged as accepted taxonomically on the basis of the listed literature under the species name. In some instances, opinions on taxonomic validity differ from author to author and users are encouraged to form their own opinion. AlgaeBase is a work in progress and should not be regarded as a definitive source only as a guide to the literature..
Names: ('C' indicates a name that is accepted taxonomically; 'S' a homotypic or heterotypic synonym; 'U' indicates a name of uncertain taxonomic status, but which has been subjected to some verification nomenclaturally; 'P' indicates a preliminary AlgaeBase entry that has not been subjected to any kind of verification. For more information on a species click on it to activate a link to the Species database):
Click here to also show infraspecific names in the list below.
Verification of data
Users are responsible for verifying the accuracy of information before use, as noted on the website Content page.
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.
Created: 23 April 2002 by M.D. Guiry
Verified by: 20 October 2011 by M.D. Guiry
Linking to this page: http://www.algaebase.org/search/genus/detail/?genus_id=44406
Please cite this record as:
M.D. Guiry in Guiry, M.D. & Guiry, G.M. 2019. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 16 September 2019.