Isthmia C.Agardh, 1832

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: K├╝tzing, 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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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 2015-12-07 by M.D. Guiry.

Taxonomic status: This name is of an entity that is currently accepted taxonomically.

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 plates.

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.

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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.

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M.D. Guiry in Guiry, M.D. & Guiry, G.M. 07 December 2015. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway.; searched on 26 September 2022

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