A model for crinoid plate circlet homologies is proposed based on a tricyclic (four-circlet) aboral cup, with Aethocrinus the characteristic tricyclic crinoid. The plate circlets in the aboral cup of Aethocrinus, from bottom to top, are lintels (named herein), infrabasals, basals, and radials. In this model, traditional interpretation is maintained for the aboral cup plates of most crinoids. Cladids, flexibles, articulates (primitively), and diplobathrid camerates are dicyclic and are composed of infrabasal, basal, and radial circlets. Monobathrid camerates are monocyclic, and they have basal and radial circlets. Disparid plate circlet homologies are reinterpreted. Among disparids, “basals” are lintels, “inferradials” and “radials” are infrabasals, and “superradials” are radials.
The “Law of Wachsmuth and Springer” is judged to be a relationship that has most fidelity applied to lumen angles. This “law” is considered to be only a consequence of development and not an invariable basis by which to determine plate homologies. In the model presented here, plates cannot be shifted around after the juvenile calyx is sutured, and arms may grow on either radials or infrabasals, whichever plates are at the top of the cup when arms begin to grow.