Sumatradinium pliocenicum Head, 1993, p.40–41, fig.22, nos.5–14; fig.23. Emendation: De Schepper et al., 2004, p.634, as Barssidinium pliocenicum.

NOW Barssidinium. Originally Sumatradinium, subsequently (and now) Barssidinium.
Taxonomic junior synonym: Barssidinium wrennii, according to De Schepper et al. (2004, p.634).

Holotype: Head, 1993, fig.22, no.11; De Schepper et al., 2004, fig.10.7–9.
Locus typicus: Erth Beds, Cornwell, SW England
Stratum typicum: Pliocene

Original diagnosis: Head, 1993, p.40
Skolochorate cysts; central body with weak to moderate dorsoventral compression and rounded pentagonal ambitus. Wall composed of two closely appressed layers and has a smooth to shagreenate surface. Periphragm thinner than endophragm and sometimes revealed by wrinkles on wall surface. Processes relatively short for genus, hollow with annular thickenings developed on inner surface of process stems, and distally aculeate. Archeopyle a rounded 2a hexa intercalary, operculum free, occasionally adherent.

Original description: Head, 1993, p.40
Central body typically light to medium brown although occasional pale, thin specimens were seen. Surface of central body smooth to shagreenate and appears finely blotched in plan view (e.g., Figure 22.13, 22.14) presumably reflecting slight irregularities in wall thickness. Processes hollow and distally aculeate (aculeae around 1.0 µm, and usually less than 2.0 µm, in length; may be recurved or flared). Processes often narrow distally and on some specimens are constricted also at base.
Annular thickenings (around 2-6, number generally increasing with process length) developed on inner surface of process stems. Processes generally unbranched but specimens occasionally seen also with one or two? bifurcating processes (Figures 22.10, 23). Observations on occasional specimens whose walls were incompletely permeated by Cellosize mounting medium suggest about 6-8 internal vacuities or tubules arranged in ring within wall at base of each process (see Figure 22.5). Process distribution apparently non-tabular, usually covering entire cyst but often concentrated around ambitus.
Dimensions:
Holotype: central body length, 73 µm; central body width, 74 µm; archeopyle width, 34 µm; archeopyle height, 26 µm; archeopyle ratio (h/w), 0.76; average process length, 9 µm; ratio of average process length/averaged diameter of central body, 0.12. Range: central body length, 54(68.1)80 µm; central body width, 57(70.2)82 µm: based on 12 specimens measured. Archeopyle width, 32(34.8)39 µm; archeopyle height, 26(28.1)30 µm; archeopyle ratio (h/w), 0.73(0.809)0.86, based on seven specimens measured. Average process length, 4.5(9.7)15 µm: based on 57 specimens measured. Ratio of average process length/ averaged diameter of central body, 0.089(0.1438)0.202: based on 43 specimens measured (see Figure 24).

Affinities:
Head, 1993, p.40-41: Sumatradinium hispidum, from the middle Miocene to Pliocene of Sumatra (Drugg, 1970) is somewhat larger (length, 78-97 µm) than the St. Erth specimens, has proportionally shorter processes which are usually aculeate, and has a granulate surface.
Sumatradinium soucouyantae from the lower through lower upper Miocene of Maryland, U.S.A. (de Verteuil and Norris, 1992) differs in having a reticulate central body surface and evexate processes. "Sumatradinium sp. "furcate spines"" of Lund and Lund-Christensen, from the middle or upper Miocene of Germany, resembles the St. Erth specimens but is larger, having a central body diameter of about 90 µm (measured from illustration).