1. 



2. 



S M 



3. 



Group /^. 

 (Holohedrism). 



Group c'J,. Group D x . 



(Pyramidal Hemihedrism). (Trapezohedral Hemihedrism) . 



5. 



Group C ] x . Group C n . 



(Hemimorphic Hemihedrism.) (Hemimorphic Tetartohedrism). 



Fig. 92. 

 Symmetry- Groups with a single Axis of Isotropy. 



And because the two operations mentioned are together equivalent 

 to an inversion, the rotating cylinder is evidently congruent with 

 its inverse image, which means that it has itself an inversion-centre. 



The group D^ possesses a single homopolar axis of isotropy A& , 

 and an infinite number of binary axes perpendicular to it. 



As the group D^ does not possess a symmetry-centre (just as in 

 the case of Z) n ), the symmetry can be also described by considering 





