ftEXAGONAL SYSTEM 



109 



in type lf>, forming the rhoml>ohe<lron of the third order. As in all 

 other tetartohcdral classes, there arc here also four new forms. 

 In the selection above, face 2 and those corresponding are taken, 

 forming t he plus right form. The selection may be so arranged that 

 1 . or 13, or 14, and corresponding faces should be selected ; 2 and 13 

 arc the plus and minus right congruent forms and 1 and 14 are the 

 plus ami minus left congruent forms. By applying the method as 

 above in turn to each of the holohedral forms of type 20, the new 

 forms of this type will be produced. 



Forms 



I. Trigonal trapezohedrons, 



r/l 



na: 



a : a : me 



n i 



KT(hkil), KT(khil),KT(ihkl), KT(ikhl). 



From the dihexagonal pyramid the four trigonal trapezohedrons 

 are derived, of which the plus right is represented in Fig. 208, and 

 the plus left in Fig. 209. There are six equal 

 polar edges, three at each pole, three long 

 and three short median zigzag edges, which 

 are bisected by the terminations of the a 

 axes. 



II. All other positions of the poles in Fig. 

 207 will yield forms already described, as 

 the trigonal trapezohedron is the only new 

 form of the type. The 

 possible forms to com- 

 bine in the type are : 



Plus and minus right 

 and left trigonal trape- 

 zohedrons, Kr(hkil), 



KT(khTl), KT (ihkl), 



KT (ikhl). 



Plus and minus rhombohedron of the first 

 order, KT (hohl), (ohhl). 



Plus and minus trigonal pyramid of the sec- 

 ond order, KT (hhihl), KT (2hhhl). 



Plus and minus right and jeft ditrigonal 

 prisms, KT (hkio) , KT (khio) , KT (ihko) , KT (ikho) . 



Plus and jninus trigonal prisms second FIQ 20Q _ The Plus 



Order, KT (hh2ho), KT (2hhho). Left Trapezohedron. 



FIG. 208. The Positive 

 Right Trigonal Trape- 

 zohedron. 



