Mastonia troglodytes (Hervier, 1897). Length, 3 mm; diameter, 1 mm. Shell: ovate, medially inflated; with two spiral granules on each whorl; yellow with chestnut brown between the granules. Spire: protoconch of four and one-half acuminate, conical whorls, the apical one and one-half whorls smooth, the others bicarinate, carinate whorls with axial threads; remaining whorls about six; suture indistinguishable. Sculpture: two spiral cords of granules with a third threadlike row intercalated on the last whorl; granules rounded on the upper whorls, axially elongate on the last whorl where the apical row is largest; base with two spiral keels. Aperture: oval; anterior canal short and recurved; no posterior canal. Color: yellow with chestnut brown staining the interspaces between the granules. These triphorids are common in beach drift and in sediments to depths of 10 m. M. troglodytes was described from Lifu.