Tursiocola alata

DESCRIPTION–LM morphology: Cells motile and apochlorotic with few to numerous highly refractive putative oil droplets (Figs 1–3). In some specimens, distinct middle cytoplasmic bridge observed between valves in girdle view (Fig. 1). Mounted specimens most often lying in girdle view with truncated oval outline (Figs 1–2, 47–51) with convex edges formed by large marginal ridges at junction of valve face and mantle, and bluntly rounded rectangular edges formed by valve apices and cingulum (Figs 1–2, 47–51). In larger specimens, marginal ridges slightly concave in middle in girdle view (Figs 47–48). Mantle areas wider at apices relative to valve middle (Figs 1–2, 47–51). Copulae porose with noticeably larger and less dense pores than valve striae areolae (Figs 47–51). Valves isopolar and narrowly linear-lanceolate with acute apices (Figs 3, 52–60). Valve measurements: length 23–40 µm, width 2.7–3.8 µm, length to width ratio 7–11, n ≥ 14. Raphe straight (Figs 52–60). Central raphe ends straight, slightly expanded, terminating just within small circular central area (Figs 52, 54, 57). Central area intersected by very narrow, linear fascia extending to edge of valve face (Figs 52, 54, 57, 59). Transapical striae slightly convergent in middle of valve becoming parallel towards apices, 24–28 in 10 µm, n = 30 (Figs 52, 54, 57, 59). Through careful focusing just above the plane of valve face, marginal ridges appear as pair of bright lines extending between the apices and along edge of valve face (Figs 52, 56). Pseudosepta evident in girdle view (Figs 1, 47–51). Pseudoseptum extends over approx. 1/5 of the valve length from each apex continuing as narrow strips along valve margin before connecting with butterfly structure (Figs 53, 55, 58, 60).

DESCRIPTION–SEM morphology: Externally, transapical striae composed of circular areolae near raphe and transapically elongated areolae on mantle (Figs 61–67, 69–74). Areolae appearing similar in outline to overlapping pin holes (Figs 64–67, 71–74), areola density approx. 30 in 10 µm. Striae on mantle shortened at valve middle adjacent to central area and fascia (Figs 62–63, 65, 70). Two large hyaline marginal ridges (width approx. 200 nm) resembling wings extending perpendicularly away from valve face (Figs 69, 71–73), one on each side of valve near valve margin (Figs 61–64, 66, 69–73). Maximum height of marginal ridge approximately same as mantle depth (Figs 61–63, 69–70). Marginal ridges decreasing in height and terminating a short distance from apices (Figs 61–64, 66–67, 69–71, 73) forming the transition between steeply sloping and relatively deep mantle and flat valve face (Figs 64, 66, 71–74). Numerous small siliceous granulae (Ross and Sims 1972) are present on mantle margin and between terminal areolae (Fig 65). Fastigii present at valve apices (Figs 64, 66–67, 70, 73–74) extending upwards towards valve face reaching approximately half of mantle height (Figs 64, 66–67). Raphe lying within a narrow axial area (Figs 64, 71–74). Polar raphe ends deflected towards secondary side of valve then apparently bifurcating around fastigium (Figs 73–74). Polar raphe end fissures extending onto the mantle and terminate near valve margin (Figs 64, 66). Internally, pseudosepta extending from apices as siliceous plates concealing structure of internal polar raphe ends (Figs 75–76) and continuing as thin strips along valve margin before widening into broad concave wings of butterfly structure (Figs 75–77). Marginal strips of pseudosepta briefly widen towards their middle, forming oblong-shaped voids constricted in the middle (Figs 75–77). Internally, raphe fissure slightly expanded in central area and positioned along center of a thick siliceous rib (Figs 75–77). Two C-shaped knobs on raphe rib on opposing sides of raphe on raphe rib at valve center; open ends of C-shaped knobs face center of valve (Figs 75–77). Epicingulum composed of open valvocopula (Figs 63, 68) with advalvar row of larger transapically elongated pores and second abvalvar row of smaller ovoid pores, pore density = 20–21/10 µm, and up to three additional abvalvar open copulae each with single row of small ovoid pores, pore densities = 33–40/10 µm (Figs 61–63, 65, 67–69). Valvocopula wide (Figs 63, 65, 67–68) with pair of opposing tabs on pars interior located at valve middle underneath butterfly structure (Fig. 68).

• Author
  Frankovich, Ashworth and M.J.Sullivan 2018
• Length Range
  23-40 µm
• Width
  2.7-3.8 µm
• Striae in 10µm
  24-28