CHARACTERISTICS OF THE SEVEN CLASSES 



379 



5. The Eedral class consists of singly terminated pyramids having faces 



in pairs. Two such pyramids may be joined base to base, pro- 

 ducing the other hedral classes. The faces of the upper pyramid 

 maybe 



6. Directly over those of the lower pyramid — Orthohedral; 



7. Alternating with those of the lower pyramid — Ameoahedral. 



The oblique position corresponding to that of the Polyaxial 

 class can not exist in this type, since the planes of symmetry of 

 the upper and lower pyramids would not coincide, violating the 

 law of Parallel Directions. 



These relations are exhibited in the following tables : 

 Table showing Forms of Classes 



Doublv terminated 



Axial 



( Faces single 



in mPn) 



Singly 

 terminated 



Axial 



Direct 



(Horizontal 



plane of 

 symmetry) 



Orthoaxial 



Oblique 



(Horizontal 



axes of 

 symmetry) 



Polyaxial 



Alternate 



(Alternating 

 axis) 



Amebaxial 



Hedral 



(Faces in pairs 



in mPn) 



Hedral 



Orthohedral 



Amebahedral 



Axial 



(Faces single 



in mPn) 



Table showing Symmetry of Classes 



Singly 

 terminated 



Axial 



Doubly terminated 



Direct 



(Horizontal 



plane of 

 symmetry) 



Orthoaxial 



Oblique 



(Horizontal 



axes of 

 symmetry) 



Polyaxial 



Alternate 



(Alternating 

 axis) 



Amebaxial 



Hedral 



Orthohedral 



A mebahedral 



Hedral 



(Faces in pairs 



in mPn) 



