Valves with apices that are sharply rounded, at less than 90°
Transversely linear chambers in the valve that have small openings externally and large openings internally
Annulae are comprised of one to four transapical striae that interrupt the typical striae at the poles. These structures are restricted to the genus Geissleria. The annulae may be distinctive or barely discernible.
The long axis of a diatom is considered the apical axis.
Valve ends that are abruptly tapered to a fine point
The araphid diatoms are pennate forms that lack a raphe system, hence lack motility. Examples of araphid genera include Diatoma, Fragilaria and Synedra.
Curved like a bow or bent along the apical axis
Perforation, or pore, in the diatom valve. The plural form is areolae.
Becoming thin or slender
A special cell that develops and expands before producing a new frustule. Maximum size of a population is restored through auxospore formation. Auxospores are usually associated with sexual reproduction.
In some taxa, an internally thickened edge of the axial area is termed the axial costa. Two axial costae may be present, one on each side of the raphe. Axial costae is a term used to refer to structures in Mastogloia smithii, among other taxa. Paddock and Kemp (1990) note that the axial costa forms a ‘gutter’, which widens slightly at the central nodule, but does not extend to the distal raphe ends.
The axial plate is a structure within some species of Gomphoneis. The term refers to the internal ‘plate’ of silica that occludes the areolae internally. The margin of the axial plate results in the longitudinal line, visible in light microscopy.