Arcuatasigma belizense G. Reid, 2018

Lobban, Christopher S. & Reid, Geraldine, 2018, New Arcuatasigma species (Bacillariophyta, Pleurosigmataceae) from Guam and Belize, and the taxonomic identities of Toxonidea challengeriensis and its variety, Phytotaxa 346 (2), pp. 169-179 : 173-175

Arcuatasigma belizense G. Reid , sp. nov. Figs 19–28

Cells solitary, plastids unknown. Length 183–210 μm, width 7.7-9.6 μm. Longitudinal striae 21–24 per 10 μm, transverse striae 18–19 per 10 μm. External areolae opening to apically oriented slits, except V-shaped slits commonly joining two areolae. Raphe diagonally bisecting the main body of the valve becoming eccentric at poles. External central raphe fissures overlapping, situated in a clear hyaline area. Internal central raphe endings within a central nodule, flanked either side by central bars.

Type:— BELIZE. Half Moon Bay, sublittoral sediment. RAF sub aqua team, 1979. Holotype BM-BP168! Half Moon Bay , Belize #24 (shown in figs 21, 26, 27) ; Paratype BM-BP239! Half Moon Bay , Belize # 34 (shown figs 19, 23, 24) ; Paratype BM-BP172! Half Moon Bay , Belize #28 (shown in figs 20, 25, 27, 28) .

Etymology:— With reference to the type locality.

Description:— This species had a sigmoid frustule with one pole twisted 180° to give the arcuate appearance of the genus ( Figs 19–21, 27). The valve had a shallow mantle. Areolae loculate ( Fig. 28), arranged in longitudinal and transverse rows of striae ( Figs 23–24). The spathulate apex was broad, with ca. 20 areolae in the striae. External areolae opened to apically elongate slits except in the occasional case of transapically joined pairs of areolae in which the external opening was V-shaped ( Fig. 22); this also occurs in A. addu ( Reid 2012, Plate 15, fig. 2). In one specimen the areolae appeared round, perhaps due to dissolution or being at an early stage of development so the openings have not been fully filled in ( Cox & Kennaway 2004). Raphe eccentric, crossing the main body of the valve diagonally, becoming eccentric at the poles running close to the valve edge ( Figs 19, 25). Terminal raphe fissure following the curvature of the edge of the pole, terminating on the valve mantle ( Figs 25, 26). The external central raphe fissures completely overlapped ( Fig. 22) as in A. addu and A. closterioides in contrast to the two Guam species, where there was no overlap or only the pores overlapped. Raphe sternum thickening was denser on the primary side of the valve. Internal central nodule (0.9-1 μm diam.) flanked either side by central bars ( Fig. 24). Internally the raphe terminates in a helictoglossa at the pole ( Fig. 23). Girdle bands were plain ( Fig. 26). The appearance of the valve, as with other members of the genus varied greatly depending on the orientation it was lying (valve, girdle or somewhere in between Figs 19–21).