Genus Description:
Description by Van Heurck (1896, p. 466):
Frustules free or united into filaments either continuous or in zig-zag colonies. Valves elliptic, suborbicular, triangular, quadrangular, etc.; margins even or undulated, usually more or less inflated at the median portion, rarely depressed, showing elevated, obtuse processes at apices, and having also sometimes hornlike processes. Girdle face more or less quadrangular, showing distinctly processes which are not so visible in the valve face. Connective zone very obvious.
The genus Biddulphia is of vast proportions; it includes some hundreds of species and the genus requires a serious monograph written by a thoroughly competent specialist who shall have devoted a very considerable time to its examination and study.
Many of the forms belonging to the genus Biddulphia have been arranged in different genera. Genera have even been established, founded simply on the number of angles of the valve, such are Triceratium, Amphitetras, Amphipentas, Polyceratium; forms which are found at the same time, and both rhomboidal and angular in form, for example, Biddulphia rhombus (Ehr.) W. Sm., and its forma trigona Cleve (Triceratium striolatum Ehr.) show that these groups are not admissible.
Remarks from Jan Rines (website):
Differerentiation of Biddulphia and Odontella:
Biddulphia: valves thick-walled with pseudocelli on elevations, mantle vertical, external ribs or ridges with associated internal costae, spines/labiate processes central when present, with short external extension, lives attached to substrate, occasionally mixed into the plankton.
Odontella: valves are thin-walled with ocelli on elevations, mantle rounded or sloped, ribs or ridges on extenal part of valve only, spines/labiateprocesses close to elevations of margins, or in two opposite groups-long external extensions, planktonic.
Generic Characteristics. Occurring as single cells, or as zig-zag chains united by mucilage pads on the horns. Colonies may be attached to surfaces (e.g. seaweed), or be part of the plankton. Cells rectangular in girdle view, and either bi- or tripolar in valve view. Valve shape is thus lanceolate to elliptical, or triangular, and sometimes with an undulate outline. Poles with rounded, pseudocellate elevations. Valves may be segmented; the external ribs or ridges possess associated internal costae. One to several labiate processes near the valve center, with small external projections. The valve mantle is high, and vertical. Areolae are large, and visible in the light microscope. Individual girdle bands may be visible in the light microscope, ornamented with rows of large areolae. Numerous discoidal chloroplasts.
Comments. The term "Biddulphioid" informally refers to taxa which may be bi-, tri- or multiangular and which possess horns (elevations) and/or linking spines. Formally, they belong to a variety of genera in two different families (Eupodiscaceae and Biddulphiaceae). A tremendous amount of confusion has ensued from disagreements as to whether classification should be based on valve symmetry or valve structure. Recognition that a single species may exhibit more than one polarity state has resulted in movement towards a system based on valve structure. Biddulphioid diatoms within our geographic range, and which are found in the plankton (either holo- or tychoplanktonic) include Biddulphia and Odontella. Many planktonic forms reported in the older literature as Biddulphia are now placed in Odontella, and are so classified in this book. Our assignment of taxa to Biddulphia or Odontella follows Hartley (1986). See also Odontella. |
