GUIDE TO THE MINERALOGIC COLLECTIONS 19 



hedron of the normal group similar to that of the tetrahedron 

 to the octahedron. 



Fig. 56 Fig. 57 



Two other forms, the tetragonal tristetrahedron (fig. 58) and 

 the hexakistetrahedron (fig. 59) are occasionally found in combi- 

 nation. Some combinations in this group are shown in fig. 60-65. 



Fi S- G 3 Fig. 64 Fig. 6 5 



