Aulacopygus differs from Hyboclypus in the structure of its apical system. Both have apical discs that are elongate, with complemental plates separating genital plates 1 and 4. However, in Aulacopygus the complemental plates also completely separate oculars II and IV, whereas in Hyboclypus such complemental plates only partially separate oculars II and IV. It is worth noting that Kier (1962, p. 29) did not believe this difference to be of generic significance, particularly as the arrangement and number of complemental plates is variable even within one species.
Barras, C. 2006. British Jurassic irregular echinoids. Monograph of the Palaeontographical Society, 1-168, pls 1-14 (publication number 625).
P. M. Kier 1962. Revision of the cassiduloid echinoids. Smithsonian Miscellaneous Collections 144 (3), 262 pp.