Petroplacus tectum 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 : 270-271

Petroplacus tectum Pomazkina, Rodionova, Sherbakova & D.M.Williams , sp. nov. ( Figs 23–32 )

LM: Valves linear broad with central constriction, protracted rostrate ends, 32–47 μm long, 18–21 μm wide. Axial area linear, widening towards valve centre, forming a transversely elongated rectangular central area. Raphe straight; external central raphe endings direct, expanding into T-shaped grooves. Transapical striae radiate, 12–16 at valve centre, 17–22 in 10 μm at apices; irregular shorter striae at valve edge between main striae; striae composed of small round poroids, 21–23 in 10 μm. SEM: Valve face flat, turning gradually toward shallow mantle. Externally, central raphe endings direct, expanding into T-shaped groove. Terminal fissures deflected bluntly to same side. Internally, central raphe endings straight, filiform; distal endings slightly bent from centre, terminating as simple helictoglossae. Areolae occluded internally by simple volae, which touching neighboring volae forming chains for stria.

Type:— RUSSIA. Lake Baikal: Northern basin, Cape Elokhin , sand sample collected by diver in the littoral zone at st. 12, 29 June 1997 (holotype: slide 0043-BK housed at the Limnological Institute SB RAS, Irkutsk, Russia, here illustrated as Fig. 23 . Isotypes: BK! slide 0043 (st. 12), Limnological Institute SB RAS, Irkutsk, Russia; BM! slide 10053) .

Etymology:— Tectum (L.) means “hidden”.

Distribution:— Only known from the type locality; very rare.

Remarks:—Specimens of P. tectum differ from P. lizae in outline by the central constriction, the number of areolae in a stria (21–23 against 35–49 in 10 μm), in the form of the T-shaped groove and the larger valve size (32–47 in comparison with 25–35 μm).

Figures

FIGURES 23–32. Petroplacus tectum from type material. 23–25. LM views showing variation of valve size. Figure 23 is the holotype. 26–32. SEM Petroplacus tectum from type material. 26, 28, 29. SEM external views of entire valve showing raphe and striae structure. 26. SEM external view showing the constriction in the centre of the valve (arrow). 28. SEM external detail of proximal raphe endings, expanded into T-shaped grooves, uniseriate striae. 29. SEM external detail of terminal fissure, uniseriate striae. 27, 30−32. SEM internal views of entire valve showing raphe and striae structure. 27. SEM internal view showing the constriction in the centre of the valve (arrow). 30. SEM internal detail of valve centre with proximal raphe endings, volate occlusions of pores. 31. SEM internal detail of valve apex with helictoglossa. 32. SEM internal view showing intercalary striae, round pores occluded by volae. Scale bars = 10 μm (Figs 23–27), 5 μm (Figs 28−31), 1 μm (Fig. 32).