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Isthmia C.Agardh, 1832

Empire Eukaryota
Kingdom Chromista
Phylum Bacillariophyta
Subphylum Bacillariophytina
Class Mediophyceae
Subclass Chaetocerotophycidae
Order Hemiaulales
Family Isthmiaceae

Lectotype species: Isthmia obliquata (J.E.Smith) C.Agardh

Currently accepted name for the type species: Isthmia enervis Ehrenberg

Original publication:Agardh, C.A. (1832). Conspectus criticus diatomacearum. Part 4. pp. 49-66. Lundae [Lund]: Literis Berlingianus.
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Type designated in Kützing, F.T. (1834 '1833'). Synopsis diatomearum oder Versuch einer systematischen Zusammenstellung der Diatomeen. Linnaea 8: 529-620, pls XIII-XIX [79 figs].
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Taxonomic status: currently recognized as a distinct genus.

Most recent taxonomic treatment adopted: Nikolaev, V.A., Kociolek, J.P., Fourtanier, E., Barron, J.A. & Harwood, D.M. (2001). Late Cretaceous diatoms (Bacillariophyceae) from the Marca Shale member of the Moreno Formation, California. Occasional Papers of the California Academy of Sciences 152: [1]-119, 39 pls.

Description: Cells either rhomboidal or trapezoidal in girdle view and attached by massive mucilate pads to form complex branching colonies. Plastids discoid. Cells epiphytic on seaweeds, e.g. species of Polysiphonia, Ceramium, etc. Valves oval (bipolar) in valve view and with no distinction into valve face and mantle. Valves heteropolar, one pole being much more elevated than the other. Frustules slightly heterovalvate, one valve being produced into a more bulbous extension than the other and having a conspicuous pseudocellus at this pole. Valves with a massive siliceous framework more-or-less equally developed on both the outside and inside of the valve (I. enervis) or the internal framework enlarged in a 'girder-like' manner (I. nervosa). Areolae elliptical to polygonal, containing complex thick cribra. In addition simple pores penetrate the framework in various places. There are no obvious rimoportula openings externally, but internally bulbous and slightly flattened rimoportulae can be found clustered on the valves; the exact distribution has not been determined. A large flanged pseudoseptum extends inwards from near the edge of the valve mantle; this is supported by struts arising from the valve surface. Copulae complete, with smaller areolae and cribra than on the valve - both valve (below) and copula (above) are figured in Fig. f. The pars interior of the valvocopula forms a strutted 'septum' which hooks over the valve pseudoseptum. The copula associated with the epivalve of the trapezoidal cells of I. enervis has a distinct notch.

The most recent alteration to this page was made on 7 Dec 2015 by M.D. Guiry.

Comments: This is a most striking genus with many complicated features (see Round, 1984a for further detail). It is not easy to fit into the classification of the centric group (see Round & Crawford, 1981) - most authorities have placed Isthmia in the Biddulphiaceae.

Numbers of names and species: There are 26 species names in the database at present, as well as 11 infraspecific names. Of the species names, 6 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: 07 December 2015 by M.D. Guiry

Linking to this page: http://www.algaebase.org/search/genus/detail/?genus_id=44413

Citing AlgaeBase
Please cite this record as:
M.D. Guiry in Guiry, M.D. & Guiry, G.M. 2021. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 07 May 2021.

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