I. INTRODUCTION Up to the moment more than one hundred species and subspecies of Neniinae are known from Central and South America. They are incorporated into 22 genera and subgenera, almost exclusively based on characters of the shells. Only little is known of the anatomy of the animals (Binney, 1871; Binney & Bland, 1871; Baker, 1961; Hesse, 1925; Scott, 1954; Loosjes, 1957; Pilsbry, 1926; Polinski, 1922; Zilch, 1953, 1959). These authors described the radula and/or the genital organs of one or several species. Thus from eleven species the radula has been described and from five the genital apparatus. By the courtesy of Prof. Dr. W. Weyrauch we were able to study the anatomical features of a relatively large number (38) of Peruvian and Argentine Neniinae. We wish to express our most cordial thanks to him for his generosity and also to Dr. B. Hubendick, who provided us with a sample of Nenia tridens (Chemnitz) from Puerto Rico. In Thiele's (1929/31) classification the American Neniinae are grouped into three genera, Nenia, Peruinia and Temesa, based on characters of the shells and of the radula. It turned out impossible to fit the information we found, into this system as it indicates the presence of many more units. In this survey we will follow Zilch's (1960) classification, which enumerates a number of genera and subgenera, based on conchological characters, although the order will be altered according to the results obtained (conf. chapter IV). Thus conchological data of the genera or subgenera involved have to be studied in Zilch (1960), or eventually in Thiele (1929/31).