The two species of Pleurosigma described by G. Shadbolt have been critically examined in the type material with LM and with SEM for specimens conforming to the types in LM. A comparison of P. inflatum Shadbolt and P. naviculaceum Brébisson did not reveal any specific difference; the former taxon has priority while the latter is designated a synonym. The protologue of P. diverse-striatum Meister matches that of P. inflatum, but new findings validate its separate status. Pleurosigma diverse-striatum differs from other members of the genus by its areolar morphology. Its status as a synonym of P. aequatoriale Cleve will be clarified in a future paper. Pleurosigma validum Shadbolt is shown to be a synonym of P. rigidum W. Smith. Lectotypes are designated for Pleurosigma inflatum Shadbolt, 1853, Pleurosigma naviculaceum Brébisson, 1854, and Pleurosigma validum Shadbolt, 1853; and Pleurosigma diverse-striatum Meister, 1934, is typified.