Catopygus is very similar to Phyllobrissus. The type species of both genera are very similar in appearance, both having the same petal arrangement, and phyllodes with two series of pore-pairs in each half-ambulacrum; the inner pore of each pair greatly reduced in size. For this reason Lambert (1902, p. 15) and Mortensen (1948, p. 167) suggested that Phyllobrissus might be considered a subgenus of Catopygus. Kier (1962, p. 76) points out that Phyllobrissus gresslyi is slightly broader, with a more depressed adapical surface, and an obliquely truncated posterior margin, exposing the periproct adapically. In Catopygus carinatus the posterior margin is pointed, and the periproct not visible from above. However, Kier (1962, p. 76) adds that although these differences may be sufficient to distinguish these genera, there are some species that have some of the characters of both type species, making it very difficult to decide to which of the two genera to assign them.
Catopygus closely resembles Pygaulus in test shape, petal arrangement, and position and shape of the periproct. Catopygus differs in having a pentagonal peristome, wide phyllodes and well-developed bourrelets.
Catopygus is differentiated from Penesticta by its broader, more developed petals, in which the pore-pairs are closer to each other than in Penesticta (Smith & Wright (2000, p. 419) and by having four gonopores (gonopore present in genital plate 2), while Penesticta has only three (no gonopore in genital plate 2).
P. M. Kier. 1962. Revision of the cassiduloid echinoids. Smithsonian Miscellaneous Collections, 144 (3) 262 pp.
J. Lambert & P. Thiery. 1909-1925. Essai de nomenclature raisonnee des echinides. Libraire Septime Ferriere, Chaumont, 607 pp., 15 pls.
T. Mortensen. 1948. A monograph of the Echinoidea: 4 (1): Holectypoida, Cassiduloida. Reitzel, Copenhagen, 363 pp., 14 pls.
A. B. Smith & C. W. Wright. 2000. British Cretaceous echinoids. Monograph of the Palaeontographical Society, pp. 391-439, pls 130-138.
