Tursiocola varicopulifera

The frustules are narrow, broadly rectangular to constricted in the middle in girdle view with bluntly rounded ends and striated girdle bands that extend slightly beyond the valve apices (Figs. 25–29). The valves are isopolar and linear lanceolate (Figs. 30–32, 34–35) with wide mantles (Figs. 25–29), and sub-acute apices (Figs. 30–36). Length 31–57 μm, width 2.9–4.7 μm, length to width ratio 10.3–12.3. The raphe is straight and lies within a narrow axial area (Figs. 30–32, 34–35). The proximal raphe ends are straight, expanded, pore-like, and terminate within a large, circular central area (Figs. 30–32, 34–35). The central area is intersected by a narrow rectangular stauros (Figs. 30–32, 34–35) that extends down the mantle and connects with a broad hyaline area along the margin of the middle of the valves (Figs. 25–29). Also visible in girdle view when focused on the plane of the raphe is a refractive line extending from the apices to the edge of the central area (Figs. 26–29). This refractive line is likely evidence of an internal siliceous rib associated with the raphe. The transapical striae are slightly convergent in the middle of the valve becoming parallel towards the apices (Figs. 32–35), 25–28 in 10 μm. The transapical striae are shortened in the middle of the valve where they terminate before reaching the valve margin (Figs. 25–29). By focusing through the valve, pseudosepta can be seen to extend over approximately 1/5 of the valve length from each apex (Figs. 30–36) and then continue as narrow strips along the valve margin before connecting with the butterfly-like structure (Figs. 30–36). The pseudosepta and the butterfly-like structure enclose two oblong-shaped areas with slight constrictions in their middle on either side of the central area (Figs. 30, 36). The girdle is composed of two to four copulae of two different types (Figs. 25–29, 37). The advalvar copulae (i.e., valvocopulae), present in all frustules, are more coarsely striated and have two undulating rows of linear pores (Figs. 25–29). When present, abvalvar bands have relatively finer striations consisting of a single row of smaller linear pores (Figs. 26, 28–29, 37). The row of pores on the abvalvar copulae curve toward the advalvar copulae (Figs. 26, 28–29, 37).

SEM morphology:—Externally, the valve face has uniseriate transapical striae composed of fine slit-like transapically elongated areolae of varying length (Figs. 38–41). The areolae are arranged in wavy longitudinal rows, approximately 17 areolae in 10 μm along the transapical axis (Figs. 38–41, 44). The valve mantle is wide and slopes steeply without any clear transition between the valve face and mantle (Figs. 39–41, 44–45). The mantle margin is wide, heavily silicified, undulate, and expanded at the apices into spur-like extensions that expand outward from the apices and abvalvarly towards the copulae (Figs. 41, 44–45). The straight raphe lies within a narrow axial area (Figs. 38–40). The central area is large, diamond-shaped, and intersected by a narrow rectangular stauros that extends to the valve margins (Figs. 38–39, 44–45). The proximal raphe ends are straight, expanded, pore-like, and terminate slightly within the hyaline central area but before the rectangular stauros (Figs. 38–39). The distal raphe ends are apparently bifurcated and obscured by overhanging siliceous flaps that bend towards the same side of the valve at both apices (Fig. 40).

Internally, the pseudosepta extend from the apices as siliceous plates for approximately one-fifth of the valve length, which then continue as narrow strips that run along the valve margins before widening into very broad concave “wings” of the butterfly-like structure in the central area (Fig. 42). The narrow strips of the pseudosepta briefly widen towards their middle (Fig. 42). The pseudosepta and the butterfly-like structure enclose two oblong-shaped voids on either side of the central area (Fig. 42). The oblong-shaped voids are constricted in the middle (Fig. 42). Internally, the raphe lies along the center of a strong siliceous rib (Figs. 42–43). Two knob-like structures are present on the rib on opposing sides of the raphe at the valve center (Fig. 43). The center of the butterfly-like structure is apically elongated and oval (Fig. 43). The wings of the butterfly-like structure are very broad, concave, and connect gradually with the marginal strips of the pseudosepta (Fig. 43).

The copulae are differentiated into two types with wide advalvar copulae (i.e., valvocopulae) possessing two rows of transapically elongated linear pores, 22–26 in 10 μm, and thinner abvalvar copulae possessing a single row of finer transapically elongated pores, 27–32 in 10 μm, (Figs. 44–45). The valvocopula is open on one end (arrow in Fig. 46) with three advalvar tabs on each side. These tabs underlie the valve mantle (Figs. 44–45). One tab is located in the middle and the other two are located before the open and closed ends of the copula (Fig. 46).

• Author
  Frankovich and M.J. Sullivan 2015
• Length Range
  31-57 µm
• Width
  2.9-4.7 µm
• Striae in 10µm
  25-28