The morphology of the family Pleurosigmataceae Mereschkowsky is documented using light and scanning electron mircroscopy. A historical overview of the treatment of the family is given and their interrelationships explored using cladistic analysis. Four new genera are described; Arcuatasigma gen. nov., Carinasigma gen. nov., Cochlearisigma gen.nov. and Costasigma gen. nov.; fourteen new combinations are proposed; and five new species described: Arcuatasigma addu sp. nov., Arcuatasigma castracanensis sp. nov., Donkinia lipscombensis sp. nov., Cochlearisigma ohsinsina sp. nov. and Costasigma simsii sp. nov. Plagiotropis is shown to be most closely related to the keeled members of the Pleurosigmataceae and is thus included in the Pleurosigmataceae, a new tribe is proposed to reflect this relationship, Carinasigmodeae tribe nov., which contains the new sub - tribe Rhocosigmodeae sub - tribe nov. within it to reflect the relationships of Rhoicosigma and Cochlearisigma. A further two tribes are described, Toxonideae tribe nov. and Pleurosigmodeae tribe. nov. Pleurosigma transilvanicum is formally transferred here to Gyrosigma transilvanicum (Pantocsek) Reid comb. nov. as it contains all the characters of Gyrosigma not Pleurosigma i.e. striae arranged in longitudinal and transverse rows. Hyalosigma is shown to be a member of the Pleurosigmataceae and is formally transferred to the family. (Quoted from the abstract). weiterlesen