Key To Recent Species

la. Chains straight, curved or twisted, horns with claw like or narrow, flattened tips 2

lb. Chains twisted, horns short, with a more or less sharp point, cell wall lightly silicified, areolation and labiate process indistinct

H. membranaceus Cleve

2a. Chains often twisted, horns ending in claw like spines, areolation and labiate process indistinct 3

2b. Chains straight or curved, horns with flattened tips, areolae and labiate process distinct H. sinensis Greville

3a. Horns long, valve surface flat or slightly concave, apertures large and rectangular H. hauckii Grunow in Van Heurck

3b. Horns shorter, valve surface convex H. indicus Karsten

Distribution:

H. hauckii and H. sinensis—warm water region to temperate. H. indicus—Indian Ocean, Sea of Java. H. membranaceus—warm water region. How to identify: The species may be identified as whole cells, especially in chains, in water mounts. The single process and the areolation are more distinct on permanent mounts of rinsed or cleaned specimens. Remarks: EM investigations showed that the single labiate process is offset from center in H. membranaceus and H. sinensis, and apical in H. hauckii (Ross et al., 1977). The areolae of H. sinensis have well developed vela which seem to be missing in the other two species. Hemiaulus indicus has not been examined with EM.

Family Cymatosiraceae Hasle, von Stosch, 8c Syvertsen 1983

Round et al. (1990) established the order Cymatosirales Round 8c Crawford and a subclass Cymatosirophycidae Round 8c Crawford for this family, whereas Glezer et al. (1988) followed the authors of the family and retained it in Biddulphiales. Most of the species of this family are inhabitants of sand and mud flats. Brockmanniella, Cymatosira, and Plagiogrammopsis occur occasionally in plankton, and two genera, Arcocellulus and Minutocellus, belong to marine plankton and have been recorded as predominant in inshore nano-plankton (Hasle et al., 1983, p. 82).

Terminology partially specific to Cymatosiraceae: Fascia (Fig. 18)—an extension of the central area forming a hyaline (unperforated) band extending the valve, i.e., transapically.

Linking spines—marginal spines that link sibling valves in chains.

Pilus—long hair flattened proximally.

Pilus valve—valve with two pili (the other valve of a cell has a process = heterovalvy).

Pseudoseptum—a membranous costa on the inner side of the valve (here in pervalvar direction).

Tubular process (Fig. 8)—a simple tube penetrating the valve wall;

distinguished from a labiate process with EM. Ocellulus—basically structured the same as an ocellus (see Hemiaulaceae), but with few porelli and a raised rim (EM).

The genera dealt with here are characterized by:

Cells single, in tight chains by linking spines, or in loose ribbons.

Low elevations, each with an ocellulus.

Bipolar symmetry.

One process per cell.

Heterovalvy.

One chloroplast.

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