STALKED CRINOIDS 



591 



or vestigial. Covering 

 plates and interradials 

 present. Two genera, 

 both from great depths 

 in the Atlantic — Rhizo- 

 crinus (Fig. 270), with 

 five arms and well de- 

 veloped orals (attach- 

 ment by branching root- 

 cirri); and Bathycrinus, 

 with ten arms and vesti- 

 gial orals ; attachment 

 by root-like branches of 

 stem (this is essentially 

 the same as root-cirri). 



Fam. 3. Pentacri- 

 nidae. — Stem consisting 

 of ossicles which are 

 pentagonal in section, 

 united in pairs by syzygy, 

 the upper one of each 

 pair bearing a whorl of 

 cirri and united by iive 

 bundles of fibres of petal- 

 like section with the 

 lower one of the pair 

 above it. No rooting 

 processes. Patina con- 

 sists almost entirely of 

 columns of radials, the 

 basals being almost or 

 completely hidden. Orals 

 absent, but side-plates in 

 the ambulacral grooves. 

 Two recent genera, Pen- 

 tacrinus (Isocrinus), with 

 three radials in each 

 column; Metacrinus, with 

 five to eight radials in a 

 column, but the third 



Fig. 270. — Rhizocrirms lofotensis. x 1|. 

 Wyvilla Thomson.) 



(From 



