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Class Coscinodiscophyceae Round et R.M. Crawford in Round et al. (1990)
The Diatoms: Biology and Morphology of the Genera. Cambridge.

Cells circular, triangular or quadrate, or elongate, usually symmetrical about at least two planes, one of which is the pervalvar plane. Stria pattern radial, orientated about a small boss or ring. Raphe always absent and costae and septa very rare. Rimoportules usually present. Mostly with many small discoid plastids. Sometimes connected into filamentous or zig-zag colonies by spines, pads of mucus or chitin threads.

Genera in Coscinodiscophyceae

Acanthoceras H. Honigmann (1910)
Arch. Hydrobiol. Planktonk. 5: 76.
Cells solitary and rectangular when seen in girdle view. Two divergent spiny extensions on valve angles.

Actinocyclus Ehrenberg (1837)
Ber. Berkanntm. Verh. Koenigl. Preuss. Akad. Wiss. Berlin, 2: 61.
Frustules or valves are usually seen in valve view. Valves are centric, flat or domed, with central and marginal areas composed of the same pattern. The valve mantle is shallow and appears striate. A pseudonodulus is visible at the valve margin in LM with careful focussing. There are no spines and no central strutted processes. Valve face areolae are large, equally spaced and distinctly hexagonal in shape. They occur in straight radial rows of variable length. The framework of silica is foam-like or bubbly (known as bullulate).

Aulacoseira G.H.K. Thwaites (1848)
Ann. Mag. Nat. Hist. Ser. 2, 1: 167
Frustules are cylindrical due to deep mantle of the valve and therefore Aulacoseira taxa are commonly seen in girdle view. Frustules usually form long chains but often appear as single valves or pairs of valves as the chains break during sample preparation. Areolae on the mantle are clearly visible in LM, and are arranged in straight or curved rows. Areolae are simple, round to rectangular. Spines may be present around the junction of the valve face and mantle, particularly in end cells (i.e. at the end of a chain). A furrow (sulcus) is usually visible, often forming a thickening (a 'Ringleiste') at the junction between the plain mantle edge and the areolate section. The valve view is circular and the valve face is either plain or bearing puncta which are usually arranged randomly. The nature of the spines and the arrangement of areolae on the mantle are particularly useful taxonomic features.

Cyclostephanos F. E. Round in Theriot et al. (1987)
Br. Phycol. J. 22: 345-347.
Cells are short, barrel to cylindrical in shape and occur either individually or in short chains. The valve view is most commonly seen. Valves are circular and concentrically undulating. A single row of spines lies at the junction of the valve face and margin but these are rarely discernable in LM. The valve face has radiating rows of areolae. These are usually comprised of one row at the centre of the valve face and a number of rows grouped into bundles (fascicles) at the valve edge, separated by slightly arched, raised thickened ridges (interfascicles). The fascicles of areolae continue down the valve mantle. There is no clear distinction between the valve face and mantle. More or less distinct ribs are present between the areolae rows on the inner side of the valve. There is a ring of marginal fultoportulae and scattered fultoportulae may occur on the valve face. A single rimoportula may be present. Cyclostephanos dubius is characterized by arched valves, radial striae with punctuate areolae and marginal alveolae. The valve face fultoportulae, marginal fultoportulae and spines are difficult to discern. The other two common species in fresh waters, C. invisitatus and C. tholiformis, have rather flatter valves. The former has bifurcate interstriae whilst the latter does not. However, both species are generally small and finely structured and these features are not always resolvable in LM. In C. invisitatus, the interfascicles are apparent as very fine ridges, a single valve face fultoportulae is frequently visible close to the centre and the marginal fultoportulae are often discernable at the end of every 5-7 interfascicles. Spines are only visible in the coarser valves. In C. tholiformis the areolae and interfascicles are difficult to discern in all but the coarsest valves, the features being easier to see in the larger, undulate forms than in the fine, flat valves. A single valve face fultoportulae is usually visible as a small ?lump? and the marginal fultoportulae are often discernable at the end of every 5-7 interfascicles. Spines are usually barely discernable.

Cyclotella F.T. Kützing ex. A. de Brébisson (1838)
Consid. Diat. 19
Cells are short and drum-shaped and are rarely found in chains. The valve view is most commonly seen. Valves are usually circular (sometimes elliptical) with a tangential or concentric undulation of the valve face (rarely flat). The central area of the valve face is clearly differentiated from the marginal zone. The central area is either structureless and unmarked or punctate (colliculate, ocellate or stellate in some species) and is surrounded by the marginal zone with radial striae (alveoli), radial ribs and interstriae. The interior of the valve margin is folded or chambered. Valve face fultoportulae (one or many) are often visible. Valve margins do not have a distinct ring of spines but a ring of strutted processes (fultoportulae) is often visible in LM.

Ellerbeckia (Moore) R.M. Crawford (1988)
Algae and the aquatic environment (ed. F.E. Round): 421.
Cells are large, shortly cylindrical and occur in chains. The valves are circular and robust and are essentially flat. Valves have a distinctly delineated central area with a different structure to the marginal zone. The valve face does not bear distinct areolae but has visible rows of connecting ribs which appear as radial markings in LM. The valve mantle is very thick. Tubular processes are present on the mantle and are easily discernable in LM in f. teres. The genus exhibits heterovalvy with one valve bearing ridges and the other a system of complimentary grooves.

Melosira C. A. Agardh (1824)
Syst. Algarum xiv:8 (nom. cons.)
Cells are cylindrical to subspherical, with high mantles, well developed girdles and usually form long chains. The valve face is flat or domed and is covered with small spines or granules. The valve mantle is not usually differentiated from the valve face in most species. Areolae on the mantle are barely discernible with the LM.

Skeletonema R.K. Greville (1865)
Trans. Microsc. Soc. Lond. Ser. 2, 13: 43
Cells with high mantles and well developed girdles which form filaments. The cells are joined by long marginal processes to give the appearance in LM of beads joined by fine threads. The cells are very weakly silicified and can become deformed during sample preparation. The valves are circular with convex to flat valve face. A single ring of processes occurs around the top of the mantle, associated closely with a ring of fultoportulae. S. potamos can be distinguished from S. subsalsum by its usually longer spines so that the valve faces are separated. Also S. potamos usually has domed valves while S. subsalsum has flat valves. The pseudo-sulcus is more distinct in S. potamos than in S. subsalsum.

Stephanodiscus C.G. Ehrenberg (1845)
Ber. Bekanntm. Verh. Königl. Preuss. Akad. Wiss. Berlin, 1845: 80
Cells are disc, drum or barrel-shaped, occurring individually or in chains. Typically seen in valve view as mantles are shallow. Valves are circular and range from flat to strongly concentrically undulating. A single ring of spines is present at the junction of the valve face and mantle. Fultoportulae are present (irregularly or regularly) beneath the marginal ring of spines. The valve centre is not clearly distinguished from the valve marginal zone. The valve centre has either irregular or single straight, radial rows of areolae. These become double or multiple rows toward the valve margin. The interstriae are slightly arched and are visible in LM. The presence of valve face fultoportulae is species specific. In some specimens they are clearly seen but in others they may be difficult to resolve in LM. One or a few rimoportulae occur in the mantle although these are rarely visible in LM.

Thalassiosira P. T. Cleve (1873)
Bih. Kongl. Svenska. Vetensk.-Akad. Handl. 1: 6
Frustules are often very small and thin. Valves are circular and flat. Very few features are visible in LM; often only the marginal spines and central and marginal processes can be seen. The valve face has radial striae which can be straight or sinous but these may be indistinct or invisible in LM. T. pseudonana Hasle & Heimdal 1970 is small (average ~5 µm diameter) . It has a smooth valve surface and appears almost structureless in LM. The only visible features are the marginal processes.

Urosolenia F.E. Round & R.M. Crawford (1990)

Frustules long and cylindrical with a long, fine, hair-like extension projecting from valve poles. The valve surface has irregular areolae but the markings are often difficult to resolve in LM. Indeed, owing to the delicacy of this genus, it is easily missed in LM. Isolated spines often survive cleaning but are easily overlooked.

Comments to: Diatom Key Development Team