224 Geological Survey of Canada. 



" The species of the genus Comatula now living, all of which are 

 true Crinoids, are attached while vounjy, but free in the adult 

 state. The invaluable observations of Thompson on this genus 

 will, as alreadv stated, be found at the end of this Decade. The 

 Marsupites of the Chalk which have no column were also free 

 Crinoids." 



IT. Side-arms or Cirri. 



"The side-arms or cirri are long, slender-jointed appendages, 

 attached to the column, the purpose of which does not appear to 

 be well understood. They have not yet been found on any of the 

 Lower Silurian species. Some of them are represented in the 

 figures given at the end of this Decade, in the article upon 

 Comatula." 



III. The Basal Plates. 



"The base of the cup consists of a set of plates arranged in a 

 circle on the top of the column, and in some species where they 

 are large constitutes a saucer-shaped support of the viscera, to the 

 centre of the bottom of which support the column is attached. 

 This part of the skeleton has usually been called the pelvis. In 

 nearly all the Lower Silurian species there are five basal plates ; 

 in the Upper Silurian, species with three or four are not uncom- 

 mon ; while in the Devonian those with five plates are compara- 

 tively rare." 



IV. The Sub-radial Plates. 



" These are always five, and constitute a row resting upon the 

 upper edges of the basals. They occur in the genera Palozon- 

 crimes, DenJocrinus, Porocrinus, Carabocrinus, Rhodocrinus, and 

 others. In Glyptocrinus, Heterocrinus, Thysanocrinus, Hybo- 

 crinus, and Cleiocrinus, there are no sub-radials, the rays spring- 

 ing immediately from the basals." 



V. The Rays and Radial Plates. 



" In all Crinoids there are five rays, the lower plates or extremi- 

 ties of which are included in the structure of the cup and form 

 part of the shell, while the upper portions are prolonged above the 

 body, and constitute the arms, which are generally free and more 

 or less branched. In Rhodocrinus and Glyptocrinus each ray con- 

 sists at first of a single series of three plates, sometimes called the 

 primary radials or simply the radials ; it then divides into two 

 series, called the secondary radials. In these two genera the 

 primary and secondary radials enter into the composition of the 



