410 Mr. WHEWELL on the Classification 
RHOMBOHEDRAL SYSTEM. 
Dirhombohedral Combinations are those which contain corresponding 
derivatives of R and R’.. > 
Remarks. Example and Figure. Combinations. 
Class VII. | Dirhombohedral. 
This pyramid is different Rh’, Emerald R, R’~ No. 1. pR, pR’ 
in position from pRr. Mohs II. 150. s 
No. 2. | pR, pR’, pRr 
Rh’. Emerald R3, R: No.'3. pRm, pR'm 
» ThPr50) 9a 
ee TS 
Hemirhombohedral Combinations are those in which half the faces of 
a rhombohedral combination are omitted in an alternate manner. 
These are also Plagihedral, if the remaining faces are not sym- 
metrical with respect to right and lett. 
Simple 
lass VIII. 
Cees Tt Plagihedral. 
When plagihedral faces Rb). quartz. 
are thus distributed, the for- Fe R,-{R?, RS, RY, Rs} 
res : 
i 2 {pRm, pRn} 
mula is —, or =. : 
r 1 p s Bil yp Lae 
IT. 146. 
When plagihedral faces Rh’. quartz. | No. 2. 
are es ie the for- Rr, R, ret Ri, Rs, RYR5} 
r 
mula is =, OF >. 
r ] 
,|pRm, pRn} 
pins Tee ey reo. 
IJ. 147. 
