Guide to Dictyoneis

Dictyoneis is most often found growing on sediments in tropical marine habitats. Many of the described species, however, appear to be extinct and are found only as fossils.

Cells are solitary, rectangular in girdle view, and heavily silicified. Valves are broadly lanceolate to panduriform, with subacute apices. Valve face merges gradually into mantle of variable depth. The raphe is enclosed in a prominent sternum that is conspicuously thickened internally running the length of the valve. The central nodule is slightly widened at mid-valve. Externally, the proximal raphe ends are coaxial and not expanded. The distal raphe ends are deflected toward the opposite sides of the valve. Internally, the proximal raphe ends are simple. The distal raphe terminates in a rudimentary helictoglossae.

The valves are bi-layered, with a basal layer consisting of closely spaced and raised virgae. Between the virgae are uniseriate rows of tiny pores in a quincunx pattern. The inner siliceous layer is exposed externally around the central nodule and valve apices. Elsewhere, the basal layer is covered by a net-like outer layer of silica (and hence Cleve’s name for the genus) containing round or oval pores. The pores increase in size toward the valve margins. Solid silica connections between the basal and outer layers form chambers on the outer side of the basal siliceous layer that are not strictly hexagonal.

The term “false pseudoloculate” has been proposed to describe this type of valve construction (Sims et al. 2018). External areolar pores along the valve face periphery are typically much larger than those elsewhere on valve; they are mainly circular but may appear as apically elongated slits at the valve apices. The most distinctive feature of the genus are the “cellules marginales” (Cleve 1890) or “false loculi” (Cleve 1894). These are the false pseudoloculi inside the valve mantle and are readily apparent when focusing through the valve in valve view. The location and appearance of the false pseudoloculi creates a resemblance to the partecta of Mastogloia. Careful observation in LM indicates that the chambers of Dictyoneis are within the valve, as opposed to specialized structures within the valvocopula as in Mastogloia.

The girdle consists of open bands. Each band has two rows of vertically oriented small poroids on the advalvar portion and is pitted (but not perforated) on the abvalvar surface. Living cells possess two large chloroplasts with scalloped margins that are parallel to the valvar surface. The plastids are located on opposite sides of the central nodule.

Dictyoneis also has some superficial similarity to the genus Diadema Kemp and Paddock (1989) but the latter has a longitudinal or “parietal” canal running the length of the valve margin and its valve structure is not false pseudoloculate.

The number of Dictyoneis taxa is problematic, as AlgaeBase lists 17 described species but the taxonomic status of 15 of these are stated to be unresolved or in need of further investigation. The generitype, Dictyoneis marginata, seems to be the only species recorded with any frequency.