From Williams et al., 2017:

[Volkheimeridium, Quattrocchio and Sarjeant, 2003, 136, 138.

Type species: Spinidinium lanterna, Cookson and Eisenack, 1970a (pl.12, fig.2)]


Original description: [Quattrocchio and Sarjeant, 2003]:

Cornucavate cysts of asymmetrically rhomboidal ambitus, with a strong apical horn; hyposome with well developed left antapical horn, but with right antapical horn variably, and often feebly, developed. Crests bearing short spines define a tabulation 4’, 3a, 7’’, Xc, 5’’’, 2’’’’.
Periphragm smooth to granulose or with small-scale ornamentation (granae, coni, baculae or spines).
Cingulum well marked on periblast; it is broad and laevorotatory, bordered by continuous crests.
Sulcus indicated by a shallow depression on the ventral surface. Endoblast ovoidal to spheroidal or with antapical flattening.
Archaeopyle formed by the opening of plates 3’and 2a (3’of linteloid hexa shape, 2a stenodeltaform to stenothetaform), which retain attachment to plate 4’’ (of flattened quadra shape). Operculum most often adnate posteriorly, but sometimes lost.

This genus is distinguished from Spinidinium Cookson and Eisenack 1962, as here emended, by the presence of crests delimiting most (though not necessarily all) plates; in the latter genus these are not developed, the spinose ornamentation constituting penitabular rows or intratabular clusters. Nevertheless, the two genera are evidently very closely related. This casts doubt upon the opinion of Bujak and Davies (1983) who, in their stimulating discussion of the evolution of peridinioid cysts, suggested that Spinidinium formed part of “a possible cyst lineage represented by the genera Alterbia [now Alterbidinium], Diconodinium, Isabelidinium, Manumiella and Spinidinium” (Bujak and Davies, 1983, p. 66). Instead, we would suggest that Spinidinium may have been derived from Volkheimeridium and that its relation to those other genera is questionable.
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