Explanation of Plate XXIV. 



All the figures on this plate are two-thirds natural siz<\ 

 Figs. 1 and 2. Views of a coral, a species of Ampleseus from the Knobstone 

 shales of Kentucky, in which the deposition of silica has occurred 

 along fracture lines following the septa. 

 3 and 4. Hinge views of two specimens of a brachiopod {Schuchertella 

 sp.) from the Knobstone shales of Kentucky, the first fractured but 

 unreplaced,- the second swollen with silica. 



5. A siliceous pseudomorph after a crinoid In which the details of the 



fossil have been destroyed. The specimen is Troost's type of Asterias 

 tennessea, from the Tullahoma formation. Cannon County. Tennessee. 



6. Fragment of silicified crinoid column from the Keokuk formation of 



Kentucky. All the original crinoidal material has been replaced by 

 silica and the deposition of quartz crystals in the fractures has com- 

 menced. 



7. A brachiopod from the Lower Knobstone shales of Kentucky, showing 



the usual fractures radiating from the beak but having transverse 



breaks tilled with silica. 

 S. A crinoid. the type of Troost's Actinocrinites humboldti, with the 



silica following the suture lines and occasionally traversing a plate. 



Tullahoma formation. White Creek Springs, Tennessee. 

 9 and 10. Two views of a silicified crinoid calyx with geodization starting 



in the sutures. Keokuk formation, Barren County, Kentucky. 

 11. A specimen of Caryocrinus from the Upper Niagaran shales of West 



Tennessee. The entire specimen is silicified and a second deposit 



of silica follows the suture lines or traverses the plates. 



ir.t 



