Abstract
Arcuatasigma is a diatom genus characterized by striae in transapical and longitudinal lines and a 180º twist in the frustule at one end, leading to spathulate apices that both curve to the same side. Three new species are described which differ in size and stria densities. Two, A. arenicolum sp. nov. and A. belizense sp. nov. have smooth valve surfaces and areolae opening externally by longitudinal slits, except for occasionally joined pairs with a V-shaped slit; this is typical for the genus. The other species, A. marginale sp. nov. has a distinctive pattern in the areolae, in which the orientation of the opening slit is different across the longitudinal rows and in which there is a distinct marginal row of areolae. We reconsidered the taxonomy of Toxonidea challengeriensis Castracane and its unnamed variety because of recognizing that it was described as having decussate striae. As a result, we created Toxonidisigma gen. nov., matching Arcuatasigma but having decussate striae (similar to other pairs in Pleurosigmataceae, e.g., Pleurosigma and Gyrosigma), and placed the two challengeriensis taxa in it as Toxonidisigma challengeriense comb. nov. and T. garbii sp. nov. In addition, we removed the synonymy of Gyrosigma spathulatum with Arcuatasigma [Toxonidea] challengeriensis and transferred the former to A. spathulatum comb. nov.