THE ECHINODERMATA 253 



columnals have elliptical surfaces, but the long axis of 

 the ellipse on the lower surface is at an angle to that on 

 the upper surface. Thus the axis of bending of succes- 

 sive joints gradually changes, enabling the stem as a 

 whole to move in any direction in spite of the shape of 

 the columnals. This arrangement gives the stem a 

 curious twisted appearance. Good specimens are found 

 in the Carboniferous Limestone of England and Ireland. 



Marsipocrinus (Sil.) differs in the very rapid branching 

 of each arm into four, the parts below the separation 

 being united by single interbrachial plates, so that much 

 more of the brachials are incorporated in the cup. 



Crotalocrinus (Fig. 71, c, Sil.) is a dicyclic inadunate in 

 which the arms branch with great suddenness, and all the 

 branches of one ray are united into a sort of membrane, 

 which was spread out horizontally when the crinoid was 

 feeding at ease and rolled up spirally on an alarm. The 

 stem is as broad as the crown, is composed of very wide 

 and thin columnals, and has a very rough surface. 



Cyathocrinus (Fig. 71, g, Ord.-Carb.) and Gissocrinus 

 (Sil.) are dicyclic inadunates with very simple cup, one 

 anal only intercalated, and much-branched arms. In the 

 former genus the arms branch in a rather lax and irregular 

 manner, and the anal tube is often very long; in the 



FIG 71. CRINOIDEA (continued}. 



(After Murchison; restored.) d, Taxocrinus tuberculatus (Miller), 

 Silurian (Wenlock Limestone). (After Murchison.) Br. i, First 

 brachial ; i Br., interbrachial. e, Eucalyptocrinus decorus (Phillips), 

 Silurian (Wenlock Limestone). (After Murchison ; restored.) IBr.i.., 

 First primibrach ; I Ax., primaxil; //Br.i., first secundibrach ; 

 ilBr., inter-primibrach ; A 7 , niche; V.S., visceral sac. /, Woodo- 

 crinus expansus de Koninck (Carboniferous Limestone), Yorkshire. 

 (After Roberts.) Pin. , pinnules, g, Cyathocrinus acinotubus Angelin, 

 Silurian (Wenlock Limestone). (After Bather.) h, Apiocrinus par 

 kinsoni Schlotheim, Bathonian (Bradford Clay). Cup and part of 

 stem, and of arms. (After Goldfuss.) Prox., proximal columnal: 

 I Br. , primibrachs ; 7/^r., secundibrachs. i, Rncrinus monihformis 

 Miller, Ladinic (Muschelkalk). Brachials shown on lower part of one 

 arm only. (After Goldfuss.) /', base ; Col., proximal columnals ; 

 // Br. i. , first secundibrach. (After Goldfuss. ) (All X J, except a.) 



