Petroplacus Pomazkina, Rodionova, Sherbakova & D.M.Williams, 2016

Pomazkina, Galina, Rodionova, Yelena, Sherbakova, Tatyana & Williams, David. M., 2016, Petroplacus gen. nov. (Bacillariophyta): a new genus of diatom from Lake Baikal, Phytotaxa 272 (4), pp. 267-276 : 269

Petroplacus Pomazkina, Rodionova, Sherbakova & D.M.Williams , gen. nov.

Cells solitary, plastids unknown. Valves having naviculoid symmetry and raphe system; girdle composed of two narrow open bands perforated with one row of regular small poroids. Valve face flat, turning gradually toward mantle ; linear-elliptic or broadly-linear with protracted rostrate poles. Axial area narrow, linear, widening slightly towards the centre. Raphe straight ; external central raphe endings straight, expanded into T-shaped grooves; external terminal fissures deflected bluntly to the same side. Internal central raphe endings straight, filiform ; internal terminal raphe endings slightly bent, with helictoglossae. Striae uniseriate, composed of small round poroids internally occluded by simple volae. Stigmata absent. To date, only two species are known, both from the type locality, both freshwater .

Type:— Petroplacus lizae Pomazkina, Rodionova, Sherbakova & D.M. Williams (vide infra).

Etymology:—from a combination of the related genera Petroneis and Placoneis , and the Latin word lacus, meaning a lacustrine habitat.

Remarks:— Petroplacus resembles both Petroneis and Placoneis genera in its rostrate poles, the shape of central and axial areas, the round areolae comprising uniseriate striae and in external polar raphe endings bent to the same side (see Table). The distinctive characters of Petroplacus genus are valves with protracted rostrate ends, filiform and straight internal proximal raphe endings, each of two girdle bands with one row of pores. Petroplacus differs from Placoneis in linear-elliptic or linear broad outline, protracted rostrate poles, and in proximal raphe endings—the external endings lie within T-shaped grooves (visible under a light microscope), the internal endings are filiform straight as opposed to deflected or hooked ( Cox 2003). Petroplacus differs from Petroneis in protracted rostrate valve poles, direct internal proximal raphe endings, the simple morphology of the velum, and the fine, rather than course, patterned valves.