frustule membranaceous, constricted in the middle, with well-marked folds extending from the junction line in both directions.
frustule, frus′tūl, n. the siliceous two-valved shell of a diatom, with its contents.
frustule twisted in the longitudinal axis, constricted in the middle; zone complex, with numerous divisions crossed by fine stri.
In one frustule I noticed one valve with one stigma and the other with four stigmas.
frustule in zone view curved; valves cuneate, dissimilar, the upper with a pseudoraphe, the lower with a raphe.
frustule membranaceous, elliptical, truncate, with broad ends.
frustule broad, with rounded angles; septa curved; valve ovate, inferior apex produced.
frustule lanceolate, truncate; zone with numerous divisions.
Each half of a frustule is called a valve, and the line at which these valves meet is called the suture.
frustule cylindrical; zone with longitudinal rows of fine puncta.