12 STRUCTURE OF THE CRINOIDE^. 



a line at the lower extremity, a short piece of which, when found 

 perfect, is always closely curled up, like a miniature coil of rope. 

 I think also that sometimes the attached species had their columns 

 broken off by some accident, and that the animal lived long after- 

 wards free, but with a portion still connected with the body. I 

 have seen specimens of Wiodocriims j)yriformis with from six to ten 

 inches of the column attached to the base of the cup, with the 

 terminal joint where the fracture occurred rounded, and the alimen- 

 tary canal closed, or, as it were, healed up. There does not appear 

 to be any way of accounting for this condition of the column, unless 

 upon the above supposition. 



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

 true Crinoids, are attached while young, but free in the adult state. 

 The invaluable observations of Thompson on this genus will, as 

 already stated, be found at the end of this Decade. The Marsupites 

 of the Chalk which have no column were also free Crinoids. 



II. 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 for 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 uncommon ; while in the Devonian 

 those with five plates are comparatively rare. 



IV. The Siih-radial Plates. 



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

 upper edges of the basals. They occur in the genera Palaocrinusy 



