Calomys callosus (Rengger, 1830) . Naturgesch. Säugeth. Paraguay, p. 231.

TYPE LOCALITY: Paraguay, Neembucu Dept., banks of Rio Paraguay opposite mouth of Rio Bermejo (as interpreted by Hershkovitz, 1962:172) .

DISTRIBUTION: N Argentina, E Bolivia, W Paraguay, WC to EC Brazil (after Mares et al., 1981b, 1989a).

SYNONYMS: expulsus, muriculus, venustus .

COMMENTS: Homogeneity of populations unified here should be viewed skeptically. According to Hershkovitz (1962), callosus also encompassed boliviae (see species account), callidus (see species account), and fecundus (see C. boliviae), together with species-group taxa enumerated above. Others have referenced, without explanation, muriculus (Williams and Mares, 1978) and venustus (Corti et al., 1987) as separate species, although Reig (1986) did not. Relationship and possible priority of venustus with respect to boliviae and fecundus especially merits study. Karyotype of nominate form published by Pearson and Patton (1976).