Anorthoneis dulcis Hein 1991: 269

Anorthoneis dulcis Hein 1991: 269 ( Figs 14–34 View FIGURES 14–22 View FIGURES 23–27 View FIGURES 28–34 )

References:— Hein (1991), p. 269, figs 2–21, 25–28; Tremarin & Ludwig (2008), figs 1–9, 10–24; Garcia & Talgatti (2008), figs 3–9; López Fuerte et al. (2011), figs 2–5.

Description. Valves broadly elliptical to ovoid in outline ( Figs 23 View FIGURES 23–27 , 28 View FIGURES 28–34 ). Apical axis: 14–16 μm, transapical axis: 9.2–12 μm. SV:— Externally, valve face flat to concave, becoming convex in the undifferentiated mantle, terminating in small marginal area ( Figs 23, 24 View FIGURES 23–27 ). Eccentric sternum neither raised nor thickened. The central area is asymmetric and larger on the wider hemivalve ( Fig.23 View FIGURES 23–27 ). Uniseriate transapical striae ( 22 in 10 μm) composed of circular areolae ( 20 in 10 μm) internally occluded by slightly perforated hymenes, in both the valve face and the mantle ( Figs 25, 26 View FIGURES 23–27 ). Close to the valve margin, each stria terminates with one larger, apically elongated or irregular areola ( Fig. 26 View FIGURES 23–27 ). Internally, the valve structures appear largely similar to the external ones, except for the sternum and the central area, which are visibly thicker ( Figs 25, 27 View FIGURES 23–27 ). RSV:— Externally, valve face flat to convex, curving gradually to form a wide, concave mantle terminating in hyaline marginal area ( Fig. 32 View FIGURES 28–34 ). Raphe-sternum eccentric, developed entirely within the valve face. Straight, expanded, pore-like proximal raphe endings ( Fig. 29 View FIGURES 28–34 ) converge coaxially on the asymmetric, subtriangular, in its wider part, and semi-circular, in its narrower part, central area ( Fig. 28 View FIGURES 28–34 ). Distal raphe endings similar in shape to the proximal ones terminate in roundish submarginal areas, close to the valve face-mantle junction ( Fig. 30 View FIGURES 28–34 ). Uniseriate valve face striae ( 18 in 10 μm) separated from the mantle striae by faint, hyaline, submarginal area present on the valve face-mantle junction ( Fig. 28 View FIGURES 28–34 , arrow). Mantle striae uniseriate, becoming biseriate towards the valve margin ( Figs 28, 30 View FIGURES 28–34 ). Areolae ( 20 in 10 μm) small, circular, occluded by hymenes ultrastructurally similar to those in the SV. Close to the valve margin, in correspondence of each biseriate stria, a larger, apically elongated areola is present ( Figs 28, 30 View FIGURES 28–34 ). Internally, raphe-sternum, central nodule, and submarginal hyaline area clearly thickened and raised ( Figs 33 and 34 View FIGURES 28–34 ). Proximal raphe endings hooked, deflected in opposite directions, converging on the central nodule ( Fig. 31 View FIGURES 28–34 ). Distal raphe endings terminating in vestigial helictoglossae, hardly reaching the hyaline terminal areas ( Fig. 32 View FIGURES 28–34 ).