of Edinburgh^ Session 1863 - 64 . 239 
A combination of these scaienohedra is a combination of the 
following forms of the prismatic system, — 
where a 
h-l . 
2h-k-l 
,= 2 = 
6 
l-h , 
2k -1- h 
= -2 . 
6 
ii-h 
2l-h-k 
6 ’ 
Ex .- — The scaienohedra 161 and 352, which occur in apatite, are 
inverse with respect to each other. 
A combination of these scaienohedra is a combination of the forms 
122, 534, 714 of the prismatic system. 
The scalenohedron likl itself is a combination of three hemibedral 
forms with parallel faces, viz., — 
TTccfi^y, -rraJS^y^ 
where cc^/3^, and y have the same values as above. 
12. The simple forms of the rhombohedral system, therefore, with 
the exception of the rhombohedron likk^ and the scalenohedron lihl^ 
follow the law of symmetry of the holohedral forms of the prismatic 
system. Hence it follows that the hemihedral forms also, derived 
from these simple forms, are subject to the laws of symmetry of the 
hemihedral forms of the prismatic system. 
Since the rhombohedron and scalenohedron are themselves com- 
binations of hemihedral forms with parallel faces of the prismatic 
system, it follows that the hemihedral forms derived from the 
rhombohedron and scalenohedron are combinations of tetartoliedral 
forms, with inclined and parallel faces, derived from the hemihedral 
forms with parallel faces of the prismatic system. 
These tetartohedral forms with inclined and parallel faces, de- 
rived from the hemihedral forms with parallel faces, may be denoted 
respectively by the symbols K-rrlikl and icTchkl. 
The tetartohedral form ttkIM cannot obviously exist ; and it is 
easily seen that the form kkIiM is included in Kithhl. Hence the 
2 I 
VOL. V. 
