Explanation of Plate XX. 



All of the figures on this plate are X 1.5 with the exception of 7-9. and 11, 

 which are natural size. The specimens are from the Lower Knobstone shales 

 at Button Mold Knob, Kentucky. 



Fig. 1. View of two segments of a crinoid column illustrating the five .sided cen- 

 tral canal. 

 2 and 3. Views of opposite sides of a crushed crinoid column. The princi- 

 pal longitudinal fractures correspond in number and arrangement with 

 the pentagonal central canal shown in fig. 1. 



4. Portion of a crinoid column in which the individual segments have been 



slightly pushed apart. 



5. Another fragment illustrating slipping of the segments and longitudinal 



fracturing. 



6. A crushed crinoid column with many small fractures as well a,s the prin- 



cipal longitudinal breaks. 



7. A crinoid column with the principal longitudinal fracture, and the ends 



tain a smaller amount of silica. 



8. Another crinoid column with the five main fractures and the ends filled 



with silica. In this specimen the individual segments of the column 

 were not found apart previous to silicification and hence no transverse 

 bands of silica have been deposited. Transverse views showing the 

 interior of figs. 7 and 8 are given on Tlate XIX. 



9. A crinoid column in which a great amount of longitudinal fracturing has 



occurred. Only the five principal fractures corresponding to the pen- 

 tameral symmetry of the organism are filled with distinct bands of 

 silica, although the small breaks contain thread-like masses of this 

 material. 



10. A crinoid column with irregular fractures filled with silica. 



11 and 12. Two fragments of crinoid columns which have suffered longitu- 

 dinal fracturing as well as transverse slipping of the segments previ- 

 ous to the deposition of silica. 



13. A crinoid column fractured in the manner of the original in fig. 6 and 



the spaces then filled with silica. 



14. An obliquely crushed crinoidal column with the deposits of silica segre- 



gated at the junction of fracture so as to assume a botryoidal struc- 

 ture. 

 The originals of figs. 1 to 6, inclusive, are calcareous specimens from the com- 

 pact shales in which no silicification occurs. The remaining specimens, figs. 7 

 to 14, were found either on the surface or in a zone of silicification. 



146 



