North American index fossils, invertebrates . plankton. Cretacic. 201. U. socialis Grinnell. (Fig. 1907.) subglobose, composed of numerous, slightly convex plates joined together, with channelled sutures and without distinct sur-face markings. IR, eight or nine in number, forming a rounded,slightly elevated, shield-like area. IB often of Kansas and Utah. Fig. 1906. Forbesiocrinus worty-eni, a complete calyx with arms, X%. (After Meek and Worthen, , V.) Order V. ARTICULATA Johannes Pentacrinus small, bowl-shaped, with dicycl
North American index fossils, invertebrates . plankton. Cretacic. 201. U. socialis Grinnell. (Fig. 1907.) subglobose, composed of numerous, slightly convex plates joined together, with channelled sutures and without distinct sur-face markings. IR, eight or nine in number, forming a rounded,slightly elevated, shield-like area. IB often of Kansas and Utah. Fig. 1906. Forbesiocrinus worty-eni, a complete calyx with arms, X%. (After Meek and Worthen, , V.) Order V. ARTICULATA Johannes Pentacrinus small, bowl-shaped, with dicyclic base. B and R unitedby close suture; R and lower brachials united by muscular articu-lation or by a rigid suture. IB obsolete. Costals rarely more thantwo, none of them pinnulate. Tegmen flexible, studded with small,irregular plates. Arms very numerously divided. R laterally incontact, but small, irregular plates frequently present between thecostals and distichals. Anal plates present only in the larval 568 NORTH AMERICAN INDEX Fig. 1907. Uintacrinus socialis. a, small individual with arms partly preserved,\ %; i>, lateral view of a larger calyx, X H» ^> analysis of calyx ; C, centrodorsalplate; ^, basals ; j^, radials ; (7^ Cj, costals ; Z?jZ>2, distich als ; P^F^, pB.\ ; PPxypost palmar. (After Clark.) Stages. Stem more or less 5-angled; the angle of the axial canalcorresponding with the outer angles of the stem. Cirri verynumerous. Triassic-Recent. 202. P. asteriscus Meek and Hayden. Jurassic. Readily identified from the joints of the column, which are thin,very symmetrically pentagonal, star-shaped bodies, with raysusually a little longer than wide and acutely angular at the ex-tremities; the center of each joint is minutely perforated and fromit radiate five petaloid areas to the angles of the joint. ECHINODERMA TA—CRINOIDEA. 569 Jurassic of the Big Horn Mountains and Black Hills and ofColorado, Nebraska, Utah, Idaho, Wyoming and Nevada. 203.
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