Mastonia gracilis (Pease, 1871a). Length, 4.0 mm; diameter, 1.0 mm. Shell: conic, inflated; last whorl with two granular spirals; apical spiral dark red with white granules, abapical spiral yellow. Spire: protoconch acuminate, of about four dark brown whorls, the apical mamillate, the others unicarinate with fine axial ribs extending on either side of the suture; teleoconch of six or seven inflated whorls; suture wide, deep. Sculpture: two granular spiral cords on the penultimate and apical whorls, three on the last whorl; spirals of prominent, circular beads separated by about half their diameters and each spiral separated by its own diameter from the next; interspaces between spirals with recessed axial columns. Aperture: ovate; anterior canal short, tubular; posterior canal barely notched. Color: apical row of granules glistening yellow, abapical row of glistening white beads, the interspaces stained with red-brown.Shells are uncommon in beach drift, more common in sediments at depths of from 10 to 60 m.
M. gracilis was described from the Hawaiian Islands. The shells are similar in shape and sculpture to those of M. monilifera Hinds, 1843, described from the Strait of Malacca and known also from Japan (Kosuge, 1962a), but the smaller size and dark red staining on the apical row of granules are distinctive. The holotype of M. monilifera is 5 mm in length and the shell is predominantly yellow, merely picked with red-brown between the granules on the abapical spiral.