Kier (1962, p. 196) states that the type species of Milletia and Pliolampas are very similar, both having a pointed posterior margin, broad, equal petals, that have broad poriferous zones, and a longitudinal peristome. Milletia also has only three genital pores (pore absent in the left anterior genital plate), a feature typical in Pliolampas.
Cotteau\'s artist almost invariably showed double pores in the phyllodes, whether they were single or double pored. This is yet again true in this genus (Cotteau, 1883, pl. 4, fig. 8), with double pored phyllodes shown in the type species. Kier (1962. pp. 196-197) states that this is certainly a mistake by the artist, as the phyllodes are single pored.
The type species Milletia is so similar to P. vassalli (Wright, 1855) that it may in fact be an objective junior synonym. However, without knowing more details about the floscelle development we cannot be certain. Milletia is a subjective junior synonym of Pliolampas Pomel, 1888 as advocated by Kier (1962, p. 196).
G. Cotteau. 1883. Echinides nouveaus ou peu connus. Bulletin de la Societe Zoologique de France, 7 (2), pp. 21-35, pls 3 & 4.
Duncan, P. M. 1889. A revision of the genera and great groups of the Echinoidea. Zoological Journal of the Linnean Society, 23, 1-311.
P. M. Kier. 1962. Revision of the cassiduloid echinoids. Smithsonian Miscellaneous Collections 144, (3), 262 pp.