TERMINOLOGY. . 147. 



ramids are transformed into rhombs, if, instead of R -f n, 

 2 (II + n) enters into the combination. 



2. If in . 145. iv., n' is made = n, and m' = m, the 

 combination is (P + n). (P -f n) m ; the forms are consi- 

 dered here in two different positions. The combination as- 

 sumes the aspect of a scalene twelve-sided Pyramid, Fig. 51. ; 

 the edges of combination are parallel to lines which, in the 

 simple pyramids, join the apices with the centres of the la- 

 teral edges. A form of this kind receives the name of a 

 Di-pyramid, and is designated by (P + n) m . (P 4- n) m or 

 by 2 ((P + n) ; "). The di-pyramids form part of the di- 

 rhombohedral forms, and combinations, which contain them, 

 are likewise considered as di-rhombohedral. 



Di-rhombohedral combinations occur in rhombohedral 

 Emerald. Vol. II. Fig. 150. 



. 147. HEMI-RHOMBOHEDRAL AND HEMI-DI-RHOM- 

 BOHEDRAL COMBINATIONS. 



A combination of the rhombohedral system is said 

 to be hemi-rhombohedrat, if only half the number 

 of the faces appear of some of the simple forms 

 which it contains. The combination is termed he- 

 mi-di-r7iombo7iedral 9 if one or several of the di-rhom- 

 bohedral forms constituting it, enter with only half 

 the number of their faces into the combination. 



It has already been observed (. 141.), that such combi- 

 nations are perfectly symmetrical : hence appearances of 

 this kind are by no means in opposition to the symmetry of 

 the combinations. 



The rhombohedron itself cannot assume a hemi-rhombo- 

 hedral appearance in the same way as other forms of this 

 system, because three faces cannot be distributed symme- 

 trically on two different apices. 



If in a scalene six-sided pyramid we enlarge the alternat- 

 ing faces contiguous to one of the apices, the symmetry 



