Genus Caballasea gen. nov. Type species: Caballasea segmentata sp. nov. Early Paleogene, southern Peru. Species included: Caballasea segmentata sp. nov., Pachychilus (Pachychiloides) lawtoni Perrilliat et al., 2008. Diagnosis Shell small, elongate; sutures canaliculate. Teleoconch whorls with axial ribs, rounded or erect as flanges. Aperture elongate. Outer lip not thickened; short, well-defined, very shallow anterior canal present; posterior canal absent. No varices or ventrolateral varix. Remarks In Peru, Caballasea gen. nov. is known from just one species, Caballasea segmentata sp. nov. In most characters, the Caballas Formation species resembles Pachychilus (Pachychiloides) lawtoni Perrilliat et al., 2008; a common species in Eocene fluvial red and green mudstones of the Carroza Formation of Mexico (Perrilliat et al. 2008). The Carroza Formation specimens, for which no complete apertures are preserved, may not be properly placed in Pachychilidae. All modern pachychilids, excepting Faunus ater (Linnaeus, 1758), have apertures without anterior or posterior canals (Glaubrecht and von Rintelen 2003). This absence of apertural canals is true for P. (Pachychiloides) cleburni (White, 1876) from the Cretaceous Bear River Formation of Wyoming, a species with which Perrilliat et al. (2008) compared P. (Pachychiloides) lawtoni. The Eocene Caballasea segmentata, however, which so closely resembles the Carroza Formation species, has a short and very shallow anterior canal. It is proposed herein that Pachychilus lawtoni from the Carroza Formation and Caballasea segmentata from the Caballas Formation are congeneric and that Pachychilus lawtoni be transferred to Caballasea. The two species of Caballasea are assigned to Potamididae based on the presence of an anterior canal. Etymology ’ Caballas ’, referencing the lithostratigraphic unit in which the genus is found. Occurrence Eocene, Mexico; Lower Paleogene, Cuenca Member, Caballas Formation, East Pisco Basin, southern Peru.

Published as part of DeVries, Thomas J., 2019, Early Paleogene brackish-water molluscs from the Caballas Formation of the East Pisco Basin (Southern Peru), pp. 1533-1584 in Journal of Natural History 53 (25) on pages 1566-1567, DOI: 10.1080/00222933.2018.1524032, http://zenodo.org/record/3670229