THE ORDOVICIAN PERIOD. 



359 



organized crinoids (Fig. 165, g-n). Starfishes and serpent-stars (Fig. 165, 

 o and p) are familiar modern forms. The crinoids may be likened 

 to starfishes turned face uppermost and fixed to the sea-bottom by 

 a stem attached to the center of the back. The body was gathered 



Fig. 165. — Ordovician Echinoderms. a, Comarocystis punctatus Billings; b, Lepi- 

 dodiscus cincinnatiensis (Roemer); c, Pleurocystis ftlitextus Billings; d, Glypto- 

 cystis multiporus Billings; e, Lepadocystis moorei (Meek); /, Mitrocystis mitra 

 Barrande; g, Archceocrinus desideratus Billings; h, Glyptocrinus decadactylus 

 Hall; i, Dendrocrinus polydactylus (Shumard); /, Anomalocrinus incurvus M. and 

 W. ; k, Ectenocrinus grandis (Hall); I, Iocrinus subcrassus (M. and W.); m, Cara- 

 bocrinus vancorllandti Billings; n, Hybocrinus tumidus Billings; o, Tceniaster 

 cylindricus Billings; p, Palceaster simplex Miller. Figures a, b, c, d, e, f represent 

 various types of cystoids from the Ordovician faunas. Figures g, h, i, j, k, I, m, 

 and n are crinoids. These cystoids and crinoids constituted by far the greater 

 element in the echinoderm fauna of the Ordovician. Figures o and p illustrate 

 two types of the star-shaped echinoderms or Asterozoa, the serpent stars, or Ophiu- 

 roidea, o, and the true starfish, or Asteroidea, p. (Weller.) 



into a cup-like form composed of close-fitting plates arranged in pen- 

 tamerous symmetry, and crowned by a fringe of arms variously and 

 often elaborately branched (Fig. 165, g-k). The upper surface of the 

 body cavity was in many cases closed over by a " vault " or arch formed 

 of similar plates. The whole, aptly styled the calyx, more closely 



