Order 1. Dicyclica Inadunata.—Dicyclica in which the dorsal cup primitively is confined to the patina and occasional intercalated anals, and no other plates ever occur between RR (Grade: Distincta); Br may be incorporated in the cup, with or without iBr, but never rigidly, and their corresponding ambulacrals remain supra-tegminal (Grade: Articulata); new columnals are introduced at the extreme proximal end of the stem.
Sub-order i. Cyathocrinoidea.—Tegmen stout with conspicuous orals. Families: Carabocrinidae, Palaeocrinidae. Euspirocrinidae, Sphaerocrinidae, Cyathocrinidae, Petalocrinidae, Crotalocrinidae, Codiacrinidae, Cupressocrinidae, Gasterocomidae.
Sub-order ii. Dendrocrinoidea.—Tegmen thin, flexible, with inconspicuous orals. Families: Dendrocrinidae, Botryocrinidae, Lophocrinidae, Scaphiocrinidae, Scytalecrinidae, Graphiocrinidae, Cromyocrinidae, Encrinidae (preceding families are Distincta; the rest Articulata), Pentacrinidae, including the recent Isocrinus (fig. 14), Uintacrinidae, Marsupitidae, Bathycrinidae (recent).
Order 2. Flexibilia.—Dicyclica in which proximal brachials are incorporated in the dorsal cup, either by their own sides, or by interbrachials, or by a finely plated skin, but never rigidly; plates may occur between RR. Tegmen flexible, with distinct ambulacrals and numerous small interambulacrals; mouth and food-grooves remain supra-tegminal and open. Top columnal a persistent proximale, often fusing with IBB, which are frequently atrophied in the adult.
All the Palaeozoic representatives have non-pinnulate arms, while the Mesozoic and later forms have them pinnulate. There are other points of difference, so that it is not certain whether the latter really descended from the former. But assuming such a relationship we arrange them in two grades.
Grade a. Impinnata.—Families: Ichthyocrinidae, Sagenocrinidae, and Taxocrinidae, perhaps capable of further division.
Grade b. Pinnata.—Families: Apiocrinidae with the recent Calamocrinus, Bourgueticrinidae with recent Rhizocrinus, Antedonidae, Atelecrinidae, Actinometridae, Thaumatocrinidae (these four recent families include free-moving forms with atrophied stem, probably derived from different ancestors), Eugeniacrinidae, Holopodidae (recent), Eudesicrinidae.
| Fig. 14.—A living Pentacrinid, Isocrinus asteria; the first specimen found, after Guettard’s figure published in 1761. |
Order 3. Dicyclica Camerata.—Dicyclica in which the first, and usually the second, orders of brachials are incorporated in the dorsal cup by interbrachials, at first loosely, but afterwards by close suture. IBB always the primitive 5. An anal plate always rests on the posterior basal; mouth and tegminal food-grooves closed; arms pinnulate. Families: Reteocrinidae, Dimerocrinidae, Lampterocrinidae, Rhodocrinidae, Cleiocrinidae.