JURASSIC ECHINODERMATA. 



Class CBJNOIDEA. 



Subclass DICYCLICA. 



Order INADUNATA. 



Suborder DENDROCRINOIDEA. 



Family PENTACRINID.E. 



Genus ISOCRINUS von Meyer. 



Isocrintjs knighti Springer. 



Plate II, figures 1-13; Plate III, figures la-d. 



? Pentacrinites asteriscus Meek and Harden, 1S65, Paleontology Upper Missouri: Smithsonian Contr., vol. 14 (172), 



p. 67, text fig. (not PL III, figs. 2a-b). 

 Pentacrinus asteriscus Knight, 1900, Geol. Soc. America Bull., vol. 11, p. 336. 

 ? Pentacrinus asteriscus Logan, 1900, Kansas Univ. Quart., vol. 9, p. 119, PL XXV, figs. 4-7. 

 Isocrinus knighti Springer, 1909, U. S. Nat. Mus. Proc, vol. 36, pp. 179-190, PL IV, figs. 1-13. 



Determinative characters. — Springer describes the species as follows: 



Specimens of moderate size. 



Stem smooth, long, slightly increasing in diameter d istally ; pentagonal with straight sides, except at the proximal 

 end, where for the first few immature internodes the younger joints are stellate. Internodala about 14, but varying 

 from 12 to 17 in the mature parts; distinctly crenulated at the margins; nodals not. enlarged, scarcely distinguishable 

 from the others except by the cirrus sockets; these are rather shallow, not extending to the hypozygal, or intranodal 

 joint, but usually encroaching upon the supranodal, in which case the apposed faces of these two joints are more or less 

 indented, producing a marked stellate outline. Conformably to this structure the cirri are directed upward. Inter- 

 articular pores extending to the fifth internode. Cirri in whorls of five; round, long, and slender, composed of 40 

 joints or more; the proximal ones relatively short and broad — about one-third as long as wide — tapering rapidly to 

 about half their breadth, and doubling in length in the first 8 or 10 joints, beyond which they continue uniformly about 

 as long as wide to the end; terminal claw not preserved. Angles of stem interradial; cirri radial; axial canal in stem 

 small, obtusely pentagonal, and apparently interradial in position. 



Cup forming a low cone, without any downward projection or basals or radials. Infrabasals well defined, filling 

 half the diameter of the column facet and entirely covered by the proximal columnal. Basals large, smooth, visible 

 in pentagonal outline, and in full contact exteriorly b\ their lateral faces; they form a closed ring, not protuberant 

 but flush with the plane of the radials, and about equal to them in height. Radials forming also a ring continuous with 

 basals. Primibrachs two, united by articulation apparently bifascial. Arms simple, or bifurcating once from the 

 sixteenth to the thirtieth IIBr, thus varying from 10 to 20; they are long, slender, with strongly oblique articulating 

 faces, and they extend to upward of 90 brachials. Syzygies at IIBr 3 4-4, and beyond throughout the arm at intervals 

 of about 5 to 10 brachials. Pinnules long, rounded, composed of elongate joints, 15 or more in the distal pinnules, 

 but the number in the proximal ones not observable. Disk unknown. 



Dimensions. — Heigbt of crown 65 millimeters; length of cirrus of 40 joints 32 millimeters; 

 length of longest stem preserved 140 millimeters; diameter of stem at second internode 2 milli- 

 meters; diameter of stem at tenth internode 2.5 millimeters. 



Description. — The first complete crown of this species was obtained by the late W. C. Knight, 

 of the University of Wyoming, in 1899, in the course of his investigations of the Morrison forma- 

 tion near Medicine Bow, Wyo. He found one very complete specimen and numerous stems and 

 fragments of arms on several small slabs of argillaceous limestone that lay in the debris. He 

 referred the crown to Pentacrinus asteriscus. Some additional material obtained the following 

 year has afforded several additional crowns. All the specimens were finally placed for study 

 in the hands of Springer, who finds reasons for believing that the form belongs to Isocrinus and 

 not Pentacrinus. * 



i A new American Jurassic crinoid: U. S. Nat. Mus. Proc, vol. 36, pp. 179-190, PI. IV, 1909. 



