736 W. A. T. SMITH DETERMINATION OP CRYSTAL CLASSES 



Polar axis l-fold — Monoclinic Domatic Class or Cliuohedral Group, 



Monoclinic System, 

 (6) Opposite ends of crystals like (axis not polar), and with a single 



plane of symmetry which is perpendicular to the axis of sym- 

 metry — Orthoaccial Order. 

 Axis G-fold — Hexagonal Bipyramidal Class or Pyramidal Group, 



Hexagonal System. 

 Axis 4-fold — Tetragonal Bipyramidal Class or Pyramidal Group, 



Tetragonal System. 

 Axis 3-fold — Trigonal Bipyramidal Class or (Trigonal Pyramidal) 



Group, Hexagonal (Trigonal) System. 

 Axis 2-fold — Monoclinic Prismatic Class or Normal Group, Monoclinic 



System. 



2. With more than o?ie axis having a period greater than one-fold: 



(With one vertical axis and two or more lateral axes perpendicular to it, 

 and with two or more vertical planes of symmetry intersecting 

 in the vertical axis. The lateral axes may join the mid-points 

 of opposite edges.) 

 (a) With a horizontal plane of symmetry (the lateral axes formed by the 

 intersection of horizontal and vertical planes) — Orthohedral 

 Order. 

 t With one or more singular axes. 



With one singular 6-fold axis — ^Dihexagonal Bipyramidal Class 



or Normal Group, Hexagonal System. 

 With one singular 4-fold axis — ^Ditetragonal Bipyramidal Class 



or Normal Group, Tetragonal System. 

 With one singular 3-fold axis — Ditrigonal Bipyramidal Class or 



Trigonotype Group, Hexagonal (Trigonal) System. 

 With three singular 2-fold axes perpendicular to each other — 

 Orthorhombic Bipyramidal Class or Normal Group, Ortho- 

 rhombic System. 

 ## With no singular axis, but with three like axes perpendicular to 

 each other. 

 The like perpendicular axes 4-fold — Hexoctahedral Class or Nor- 

 mal Group, Isometric System. 

 The like perpendicular axes 2-fold — Dyakisdodecahedral Class or 

 Pyritohedral Group, Isometric System. 

 {!)) With no horizontal plane of symmetry (the vertical planes alternat- 

 ing with the lateral axes) — Amedahedral Order. 

 # With a singular axis of symmetry. 



The singular axis 3-fold — Ditrigonal Scalenohedral Class or 



Rhombohedral Group, Hexagonal (Trigonal) System. 

 The singular axis 2-fold — Tetragonal Scalenohedral Class or 

 Sphenoidal Group, Tetragonal System. 

 ## With no singular axis, but with three like 2-fold axes perpen- 

 dicular to each other; also with four like 3-fold axes. Some- 

 times one set, sometimes the other is more prominent — Hex- 

 tetrahedral Class or Tetrahedral Group, Isometric System. 



