The Holectypoida are restored to the rank of an order of the Echinoidea Irregularia. A classification, somewhat modified from that proposed by Gregory (50), is given, and revised diagnoses of the families, subfamilies, and genera are drawn up. A comparative study of the morphology of the skeletal structures of typical genera of the group is given; and, in the light of the results of this study, the course of evolution both within and beyond the limits of the order is indicated.
The Holectypoida are regarded as an annectant group of the Irregular Echinoids, whose characters retain a considerable uniformity owing to a persistent retardation of evolution. At various periods offshoots from the Holectypoid stock appeared, which, usually with a relatively accelerated differentiation, developed into the various orders and families of the Irregularia. The order commenced in the Liasic period, and became extinct at the end of the Cretaceous. Two of the groups of Echinoids now living retain many features that were characteristic of the Holectypoida (the Echinoneidæ and the Fibulariidæ). The other groups of Irregular Echinoids show a much greater departure from the primitive character, but they all possess some features which indicate their Holectypoid anptzy.
Two new names are introduces in the Systematic Part:—Megapygus as a subgenus of Pygaster, corresponding with the Pygaster (sens. str.) of Pomel (37), with type M. umbrella; and Conulopsis, a genus including the “Echinoconus” of Desor (21), with type C. roemeri d'Orbigny. The latter group will be studied in greater detail in a forthcoming paper.