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Crinoids are marine invertebrates of unsurpassed variety of form and complexity of skeletal organization. They range from Early Ordovician time to the present, but their development in kinds and numbers culminated during the Paleozoic.

The evolution of fossil crinoids is an especially interesting study, because modification in different directions may be observed in each of several structural features. Also, evolution along one line may be accelerated greatly as compared with others. Investigation of the crinoids has special value in elucidating principles of evolution, and conclusions as to the nature of evolution among these organisms provide a basis for a classification that reflects their phylogenetic relations.

This paper summarizes extensive researches on Paleozoic crinoids, giving our conclusions as to (1) the natural main divisions among crinoids, (2) the significant evolutionary trends shown by each main group, and (3) a revised classification of Paleozoic crinoids of the world, based on characters that express the inferred most important evolutionary changes.

Analysis of fundamental structures of the different types of Paleozoic crinoids leads to recognition of three chief divisions, which are designated as the subclasses Inadunata, Flexibilia, and Camerata. This grouping accords with conclusions of Wachsmuth and Springer rather than those of Bather. Each subclass contains two orders, which in the inadunates and camerates are defined primarily on the presence of one circlet of plates or two circlets below the first ray plates, or radials. These orders are new: Disparata and Cladoidea in the subclass Inadunata, and Monobathra and Diplobathra in the subclass Camerata. The Flexibilia contains the orders Taxocrinoidea and Sagenocrinoidea, named by Springer.

Each subclass of Paleozoic crinoids is represented in Middle Ordovician rocks and each persists into Upper Permian beds. They run closely parallel in geologic distribution and they show several common evolutionary trends. (1) Among the Inadunata, evolution affects chiefly the form of the dorsal cup; the number of plates in the lowermost circlet, which reduce by fusion; structure of the posterior side of the cup, including the number and arrangement of so-called anal plates; characters of the anal sac; the nature of articulation of the arms; and structural organization of the arms. Primitive cups of steep-sided conical form are gradually modified to low discoid shapes, concave at the base. The lowermost plates of the cup are reduced from five to one. The anal plates and sac are identifiable in many inadunates as derivatives of the right posterior ray; and as evolution progresses, the anal plates tend to be expelled from the cup. Nearly smooth articular surfaces of ray plates, indicating weak mobility, become strongly ridged and grooved for attachment of muscles and ligaments, indicating great mobility. The arms tend to branch in equal rami, but advancement leads to abundant unequal bifurcations also, and this is followed ultimately by regressive evolution that trends toward simplicity of arm structure. End products of evolutionary change are a low cup having perfect pentamerous symmetry, anal plates lacking in the cup, anal sac weak or absent, and arms having branchlets (pinnules) but otherwise simple. (2) Evolution of the Flexibilia follows much the same course as in the Inadunata, tending to develop perfect pentamerous symmetry. The differentiation of arms and cup becomes obscure in many of these crinoids. (3) Primitive camerate crinoids are distinguished by prominent median ridges along rays in the cup, depressed interray areas composed of very numerous small plates, the posterior interray wider than the others and bearing a median ridge, the lowermost ray plates (radials) laterally separated all around by interray plates, and several plates in each ray incorporated in the cup. Evolution leads to progressive reduction in the number of plates of the lowermost circlet of the cup, obliteration of the median ray ridges and interray depressions, enlargement and upward displacement of interray plates, and reduction in the number of ray plates of the cup. An intermediate evolutionary stage is marked by lateral contact of the radials with one another except at the posterior side, where an anal plate intervenes; an advanced stage is defined by upward displacement of the anal that brings the radials into lateral contact all around. Ultimately all interray plates, including anals, are expelled from the cup, and the arms are free above the radials. At least four different stocks of camerates follow closely parallel courses of evolution that produce homeomorphic end products having simplified structure and perfect or nearly perfect pentamerous symmetry.

Similarities in the evolutionary trends of widely different groups of crinoids provide basis for conclusions as to causes and directions of evolution. This comparative study has value in defining principles of evolution.

The classification of Paleozoic crinoids offered in this paper covers all known genera of the world, although some are not definitely allocated. Among Inadunata, 14 families containing 64 genera are included in the order Disparata, and 38 families containing 223 genera are included in the order Cladoidea. One family and genus of inadunates is unassigned to an order. Of the 53 inadunate families recognized, 14 are new. Among Flexibilia, 6 genera are placed in the one family of the order Taxocrinoidea, and 42 genera are grouped in 5 families of the order Sagenocrinoidea. Among Camerata, the order Diplobathra contains 8 families and 34 described genera, and the order Monobathra contains 21 families and 94 described genera; of these 29 families, 2 are new. Unassigned to subclasses are 3 families, each consisting of a single genus. Unclassified genera, including unrecognizable forms, are 51 in number, making a total of 518 genera that are assigned to 91 families or unclassified. One new genus, Plummericrinus, from Pennsylvanian and Permian rocks of the midcontinent region of the United States, is described; it is a dicyclic inadunate crinoid.

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