Explanation of Plate XXI. 



Figs. 1-3, and 8-11, are X 11, while the resl are natural size. All of the 

 specimens were collected in the Lower Knobstone shales at Button Mold Knob, 

 Kentucky. 

 Figs. 1 and 2. A geode with a crinoid column as a basis. 



3. A small geode with small fragments of the crinoid column still show- 



ing on its surface. 



4. A larger example retaining little trace of crinoidal matter. 



5. The interior of a specimen similar to fig. 3, lined with drusy quartz. 



6. A geode similar to fig. 2, with the crinoidal calcite removed. 



7. Another geode derived from a much fractured crinoid column. 



8 to 11. Four small geodes showing their derivation by deposition of 



silica in crinoid columns. 

 12 and 13. Top and side views of two small geodes whose formation from 



crinoid columns is apparent from their five-sided outlines. 

 14. A mass of crinoidal fragments showing silica replacing the larger 



fractures. 



148 



