The pleurodire genus Chelus contains three species: the extant type species C. fimbriata (Schneider, 1783 ), which lives in most major drainages in northeastern South America (Fig. 1 ); the late Miocene C. lewisiWood, 1976 , from the Urumaco Formation of Venezuela (Wood, 1976 ); and the middle Miocene C. colombiana (Wood, 1976 ), from the Villavieja Formation of Colombia. C. colombiana also has been reported from upper Miocene strata at Estado do Acre, Brazil (Bocquentin, 1988 ; Bocquentin and Rodrigues dos Santos, 1989). Chelus species are differentiated by shell features, because skulls are not known for either fossil congener. Sanchez-Villagra, Linares et al. (1995) examined the diagnostic utility of shell features by performing a morphometric analysis of all three Chelus species. They concluded that although the shape and size of the first neural and entoplastron were too variable to be reliably used to differentiate species (contra Wood, 1976 ), other features such as the shape and size of the carapace, the relative heights of knobs along the dorsal ridges on the carapace, the proportions of the bridge, and the position of the intergular scale were reliable. No previous workers have considered whether the shapes and positions of scars for contact of the axillary and inguinal buttresses on the ventral surface of the carapace and of the ilium, ischium, and pubis on the dorsal surface of the plastron are useful for differentiating species of Chelus.
