Fragilariforma horstii

Frustules rectangular in girdle view and only partially seen due to prominent central swelling, forming chains with the aid of spines (Figs 11, 16, 17). Valves isopolar, cruciform with narrowly rostrate, tapering apices (Figs 1– 5, 7–10, 12, 13). Length 4 –15 μm, breadth 4 –12 μm, striae density 30 – 38 in 10 μm (n = 79). Sternum very narrow and straight, faint in LM (Figs 1– 5,7–10, 12, 13 and 14). Valve mantle steep (Figs 14 – 21) with edge parallel to the valve face/valve mantle border (Figs 14, 15) and devoid of ornamentation at the valve apices (Figs 14, 15). Striae on each side of valve axial area aligned opposite to one another, curved on valve face and mostly radial throughout the valve (Fig. 14). Striae uniseriate, some uninterrupted from valve face to valve mantle (Figs 14, 16), but some others interrupted by a spine located on the valve face/valve mantle edge (Fig 14). Some striae shorter and not ending at the valve axial area, especially those located in the valve central expansion (Figs 14, 15, 17, 19). Volae not seen. Spines solid, conical becoming spatulate at the ends, and surrounded at the base by a ring of silica with projections (Figs 14, 16, 17). Spines mostly on costae, but sometimes along a stria at the valve face/valve mantle edge (Figs 14, 16, 17). Costae raised only on the valve interior making striae appear as troughs (Figs 18 – 21). Apical pore fields present, with several round poroids irregularly disposed at the valve mantle (Figs 14 –16, 18, 21). One portula per valve and represented by a round hole, isolated, and located in the place where a striae would have developed (Figs 15, 18, 21). Girdle bands wide, without perforations (Fig. 19). Plastids not observed.

• Author
  E.Morales, Manoylov and Bahls 2012
• Length Range
  4-15 µm
• Width
  4-12 µm
• Striae in 10µm
  30-38