Literature published after 1900 contains many scattered descriptions of new color forms and subspecies. Of the 309 names in the nomenclatural list, 111 (35.9%) were published after Pilsbry (1900). Adolf Michael Zilch (1953) listed type specimens in the Senckenberg Museum, and illustrated many previously unfigured species. Frank Fortescue Laidlaw & Alan Solem (1961) recognized 74 species by name, and considered that material from the Banda Islands probably represented an undescribed species. Eleven of the species recognized by Laidlaw & Solem were described after the appearance of Pilsbry's monograph. However, several species recognized by Pilsbry have subsequently been subordinated to subspecific or varietal status, and a few names have been transferred to incertae sedis, since they are based on hundred-year-old references that have not been substantiated by more recent collectors. In fact, the study by Laidlaw & Solem (1961) forms a supplement to Pilsbry's monograph with his extensive plates, and many of Laidlaw & Solem's conclusions concerning the relationships of color forms described as species were taken not so much from new samples, but from the extent of variation that was outlined by Haniel (1921) in his pioneer study.