Explanation of Plate XIX. 



All of the figures on this plate, except 3, <>, and It, are slightly enlarged. 

 Fig. 1. A fragment of the cephalopod shell Orthoceras du&eri broken through the 

 middle and showing the chambers lined with dog tooth spar. 



2. The wall of one of the crystal lined chambers viewed from above. 



3, 4, and 5. Natural size view of the brachiopod PlatystropMa Jijn.r and en- 



larged views of its halves, showing the geode nature of the shell. 

 6, 7, and 8. The brachiopod Rhynchotrema capaw, natural size, and enlarge- 

 ments of its halves. In this case the geode occupies only a part of the 

 shell, the remaining portion being filled with indurated clay. 

 The originals of figures 1 to 8 are from the Ordovician rocks of south- 

 western Ohio. 

 9. Basal view of a crinoid calyx, natural size, in which the deposition of 



silica in the plate sutures has not progressed far. 

 10. View of the cross-section of the original of fig. 7, Plate XX. The interior 

 of this specimen is almost completely filled with quartz crystals. In 

 this as well as in figs. 11 and 12, the dark colored material is of un- 

 changed crinoidal calcite. 

 11 and 12. Transverse views of the original of fig. 8, Plate XX, and of 

 another specimen when fractured. The five silica-filled fractures and 

 the hollow central area are characteristic. 

 144 



