Explanation of Plate XXXI. 



Fig. i. Partially twinned crystal of Fig. 2. Twin crystal of Joplin Calcite. 

 Joplin Calcite. Twinning plane e,-}£ R. 



University of Chicago collection. University of Chicago collection. 



Fig. 3. Same crystal as Fig. 2, drawn in 

 the position of the negative scale- 

 nohedron. 



