STRUCTURE OF THE CRINOIDE^. 11 



different works noticed ; and to collect so great a mass of authority 

 must have cost a vast amount of labor. 



The above are the most important works containing information 

 relative to the structure of the Crinoidece founded upon the per- 

 sonal observations of the authors. The list of books, or periodicals 

 with articles or papers describing new species or genera, is too 

 extensive to be given in this place; and I shall therefore now 

 proceed to notice in detail the different portions of the skeleton of 

 the Crinoids, and to explain the meaning of the technical terms used 

 in the subsequent portion of this memoir. 



I. The Column or Stalk. 



The column usually consists of a long and slender cylindrical stalk, 

 composed of numerous short joints, so closely articulated together, 

 that, during the life of the animal, it must have possessed a very 

 considerable amount of flexibility. It seems probable that in species 

 where the joints are alternately large and small, as in Glyptocrinns, 

 there was a greater degree of pliancy than in those instances where 

 it is formed of thin, equally large circular plates, as in the lower 

 part of the appendage in Rhodoainus pyriformis. In the Corniferous 

 limestone smooth round columns one inch in thickness are often 

 found, and these are so firmly constructed, that they must have stood 

 upright, supporting the body of the Crinoid, as upon the top of a 

 pillar. The columns are either pentagonal throughout their whole 

 length, or pentagonal in one part and round in another, or altogether 

 round and smooth. In all the species they are perforated from top 

 to bottom by a small central canal, which is also either circular or 

 pentagonal. This canal no doubt served the purpose of conveying 

 the nourishment from the interior of the body to every part of 

 the column, by which its growth was provided for. In nearly all 

 Crinoids the lower extremity of the column was attached to the 

 bottom of the sea or some other solid object, such as pieces of 

 floating tim/ber, either by a number of branching rootlets, as in 

 Rhodocrinus pyriformis, or by a broad, solid base, as in Cleiocrinus 

 regius. 1 think however that certain Lower Silurian species were 

 free, and moved about through the water, dragging their columns 

 after them. I have seen at least a hundred columns of Glyptocrinm 

 ramulostis with the lower part preserved, and could never discover 

 any signs of an attachment. In this species the column at the upper 

 end is often half an inch in thickness, and it tapers gradually to half 



