Mortensen (1948, p. 109) considered Ressopygus a synonym of Galeropygus. Kier (1962, p. 25) points out that the type species of both genera are very similar, differing only in that petals I and V distally curve posteriorly in R. constantini, whereas in G. agariciformis (=G. sublaevis) they curve anteriorly. Kier (1962, p. 27) thought that such a minor difference in not sufficient to warrant generic distinction.
P. M. Kier 1962. Revision of the cassiduloid echinoids. Smithsonian Miscellaneous Collections 144 (3) 262 pp.
T. Mortensen 1948. A monograph of the Echinoidea: 4 (1): Holectypoida, Cassiduloida. Reitzel, Copenhagen, 363 pp., 14 pls.
Barras, C. G. 2006. British Jurassic irregular echinoids. Monograph of the Palaeontographical Society no. 625 (part of volume 159 for 2005), 1-168, pls 1-14.