190 THE CEINOIDEA CAMEEATA OF ]^OETH AMEEICA. 



posteriorly in most of the species of the Thysanocrinid^. In the new genus 

 Idiocrinus, in which there is but one interbrachial plate to each of the ^yq 

 sides, there is no special anal plate, and the posterior interradial, which is 

 larger and rests upon the basals, serves as the anal. 



The fomily is confined to the Silurian, and is composed of six genera, 

 with twenty-three species, of which sixteen are from America, and seven 

 from the upper Silurian of England and Sweden. 



THYSANOCRIlSrUS Hall. 



1836. Dimerocrinus — Phillips ; Murchison's Silur. Syst., p. 674, Plate 17, Pigs. 4 and 5. 



1841. Bimerocrinus — ^\ijA.-m.\ Monatsber. Berlin. Akad. d. Wissenscli., p. 208. 



1850. Dimerocrinus — d'Orbigny; Prodrome I., p. 46. 



1852. GlT/ptaster — llKLj.; Palseout. N. York, Vol. II., p. 187. 



1852. Thysanocrims — Hall; Ibid. p. 190. 



1852. Dimerocrinus— Vi'OKmQ^^^ ; Cours element., Part II., p. 142. 



1855. Dimerocrinus — ^(^^M^^; Letlisea Geognost. (Ausg. 3), p. 237. 



^^'^- j ItZZtus ] - ^''^^^ ' T^^^*^ ^' P^^^°^^^-' ™- I^" PP- ™ ^-d 318, Plate C, Pig. 13. 



1859. Dimerocrinus — M.'u^cBmo^ ; Siluria (3d ed.), p. 535, Plate 13, Figs. 4 and 5. 



^^^^' { ^mmZocTnZ I "~ ^^^-^^^^^^ ^^^^ HuP^ 5 ^ist. Natur. des Zoophytes, pp. 128 and 131. 



1863. Glyptaster — HkiA.; Trans. Albany Inst., Vol. IV., p. 202. 



1873. Dimerocrinus — Salter; Catal. Geol. Museum, Cambridge, p. 120. 



1878. Gly pf aster — Uklj. ; 28th Rep. N. Y. State Mus. Nat. Hist. (ed. 2), pp. 131 and 133. 



1878. Eucrinus — Angelin; Iconogr. Grin. Suecise, p. 24. 



{Thysanocrinus \ 

 mueZZnu. \ - ^"™" ■' "='"'^^- ^- ^'''^°"'-' ™- I-' PP- 368 ^-^ ^^S- 

 Eucrinus / 



\'BimlZ'crinuX~^' ""'' ^'"^ ' ^«™'"i Palfeocr., Part II., pp. 193 to 199. Also ibid., Part 

 ^881- \Eucrinus f HI-, P- 101. 



V Thysanocrinus (syn. of Dimerocrinus'). 

 r Glyptaster ^ 

 1889. -j Eucrinus |- _ S. A. Miller ; N. Am. Geol. and Palseontology. 



' Thysanocrinus ) 



Syn. Dit?ierocrifi2is Phillips. 

 Syn. Glyptaster Hall. 

 Syn. Eucrinus Angelin. 



Calyx subglobose, urn or bell-shaped ; the rays marked by a more or less 

 conspicuous ridge; the general surface of the plates smooth or variously 

 ornamented. Infrabasals five, small, barely protruding beyond the column 

 or entirely hidden by it. Basals five, four of them equal, angular above ; the 

 fifth truncated, and supporting a large anal plate. Eadials considerably 

 larger than the costals, their lower sides distinctly angular, the lateral faces 

 comparatively short. Costals two. Arms ten or twenty, rather strong and 



