of Crystalline Combinations. 393 
hedral, oblong-pyramidal, square-pyramidal, and rhombohedral 
systems respectively. 
Let pO, pP, pQ, pR, represent forms derived by the first law 
from each of these forms, according to the index p. 
Let pOm, pPm, pQm, pRm, represent forms derived from pO, 
pP, pQ, pR, by the second law, according to the index m. 
In the rhombohedral system, if R be turned round its axis 
through half a circumference, it is said to be in a éransverse 
position, and is designated by R’. And the derivatives of R’ are 
marked pR’, pR’m, &c. 
In the oblong-pyramidal system, if one diagonal (WC, Fig. 2.) 
be called the principal diagonal, the other (7B) is called the 
transverse diagonal. And when the derivation is made on the 
transverse diagonal, it is marked with P’, as pP’m. 
‘ 2m .. ; 
It is easy to see that mR at when m=0, is an equilateral 
six-sided pyramid, with the same axis as the fundamental form. 
1 1 : 
In the same manner mO = and mQ >? When m=0 are equilateral 
four-sided pyramids, in a diagonal position to O and Q: mP — 
when m=0, is a prism with its axis in the direction of the principal 
diagonal. 
These. forms are, for the sake of abbreviation, marked thus, 
Rr, Or, Qr, Pr 
Sometimes one half of the number of faces resulting from any 
law is omitted, according to some principle of alternate selection. 
In this case the former is said to be hemithedral, and is indicated 
by writing h before its symbol. 
Hemihedral forms are sometimes plagthedral, arith the letters 
rand | are used to indicate the obliquity to the right or the left, 
as will be shewn. : 
