The notion that a parallel-sided slit in the outer lip of a shell indicates the presence of two gills has been a significant concept in the classification of Paleozoic gastropods. However, a deep backward-curving apertural notch may also indicate two gills. The Ordovician taxa Lesueurilla and Lecanospira have such a notch and therefore were pleurotomariaceans, in at least a functional sense. Historically they were never judged to be Pleurotomariacea and if they are transferred, the suborder containing this superfamily should be expanded and recast. Removal of these genera from the Euomphalacea and Macluritacea, respectively, further weakens any assignment of both of those superfamilies to the same suborder. The Euomphalacea may have been derived from Lecanospira-like pleurotomariacean gastropods. The Macluritacea have been reduced in content by other recent investigations and are judged even more restricted an offshoot of the main stock of gastropod evolution than they were in past classifications.--Modified journal abstract.