Morphological Studies on the Echinoidea Holectypoida and their Allies

Extract

Liassic Irregular Echinoids are rare. The interest attaching to them is exceptionally great, and is enhanced by this rarity. Separation of Exocyclic from Endocyclic types can hardly have preceded the Rhaetic period; hence each Irregular form found in the Lias might be expected to display evidence as to the origin of its sub-class. It cannot be claimed that the few such genera known fulfil expectation. Plesiechinus (represented by P. reynesi) occurs in the “Charmouthian”; it is but little removed from “Regularity”, but the Bajocian species P. ornatus (whieh is locally abundant) seems “just as good” in that respect. Holectypus appears in the Toarcian with H. conquensis; its occurrence at this horizon gives a striking illustration of morphogenetic acceleration, and there by negation to hopes of “genealogical” evidence. Species of Galeropygus are recorded from the Upper Lias; orthodoxy would probably repeat the preceding sentence here, but vide infra. The case of Collyrites, recorded from deposits of Liassic age, must surely be compared with that of Holeclypus; for every feature of the Collyritidae is in violent contrast with “Regular” structures.