82 



in a softer state than it afterward assumed. In this way the base 

 increased in size with the growth of the auimal, and was made to 

 fill the inequalities of the surface, to which it attached, and to 

 extend over the border so as to form hooks or anchors of sup- 

 port. The nutrition for the formation of the organic structure of 

 the base was furnished in the same manner that it was supplied 

 for all other parts of the skeleton of a crinoid. The pores of the 

 base were channels for nutrition and were appropriated exclusively 

 to the construction and support of it. 



The plates of a crinoid column were enlarged with the growth 

 of the animal, as bones and shells are increased in size; but new 

 plates seem to have originated exclusively at the lower end, or 

 within the obconoidal cavity, in the base, at the end of the col- 

 umn; none appear to have been intercalated between older plates 

 and none were added at the superior end of the column. The 

 columnar canal was, therefore, a channel for nutrition, and noth- 

 ing passed into it except the digested and reparatory juices for 

 the columnar cords or tendons and the skeletal plates and base. 



The base illustrated is from the Hamilton Group, at Louisville, 

 Ky., but it does not differ in organic texture or structure from 

 bases found in other groups of rocks. 



