Explanation of Plate XXIII. 



All the views on this plate are natural size and are <>f the brachiopod Athyris 

 lamellosa L'Eveille, from the Lower Knobstone shales of Kentucky. 

 Fig. 1. A normal example (after Hall and Clarke). 



2. A flattened example, showing the main fracture line at right angles to 



the beak. 

 3 to 5. Three specimens (foreshortened) with the hinge area lengthened 



by crushing. 

 6 and 7. Two specimens distorted by oblique pressure. The main fracture 



lines coincide with the oblique dissection of the shell. 

 8. An example with the hinge length greatly shortened. 

 The above specimens were collected in the impervious shales and conse- 

 quently the fractures are not filled with silica. 



Fig. 0. An example from a water channel in the shale. This shell is so swollen 

 by silica that the length and thickness are about equal. 

 10. A specimen still further enlarged by the deposition of silica. 

 11 and 12. Two siliceous geodes showing their derivation from the Athyris 

 by deposition of silica in the original shell. In this case the cal- 

 careous shell fragments between the siliceous ridges have been worn 

 away. 

 152 



