J. D. Dana on Lettering figures of Crystals. ‘403 
accent, and those to the obtuse angle, none, (or the reverse, if the 
fundamental form be oblique from an acute edge). The follow- 
ing vertical series of planes will be observed in figure 6, an octa- 
hedron formed on the angles of the prism represeuted in figure 5. 
The positions of the planes are indicated by the place of the sym- 
bols, the planes themselves not being represented. ‘The letters 
m,n, here used, stand for any numerals that may occur. : 
m-6 m-i m. mn m-6 mn m! m'-r 
0-6 o~n o o-n 0-6 o'-n o o-% 
—m-o —m-n —m —m-n —m-6 —m'-n —m! —m! 
The lettering for the hexagonal system (figs. 7, 8) is a simple 
transfer of the ratios as ised by Naumann, in the manner already 
explained, and farther elucidation is hardly required. In figure 7, 
8. 
Calcite. 
HEXAGONAL SYSTEM. 
the numbers 4, 1, 2, 0 in one vertical series, exhibit the relative 
lengths of the vertical axis of the planes, viz. 3a, la, 2a, xa. In 
the series 1-2, 2-2, 4-2, 0-2, the same fact is shown, the relation of 
the vertical axis for these planes being la, 2a, 4a, xa. In figure 
8, R is synonymous ‘with 1. | 
In the monometric system, (fig. 1,) the method is the same, 
except that since the vertical axis equals the lateral, no distine- 
tion is made, and the first of the two figures in a symbol does not 
necessarily refer to the vertical axis. ‘The expressions are the 
general ex ions, as used by Naumann. 
In mere son it ‘may be observed, that the letter P, employed 
by Naumann, is hardly necessary even for the written symbol, 
and the expressions would be even more intelligible in descrip- 
tion, without the use of it; for the short or long mark, referring 
to the shorter or | ateral axis, and the accents, are here 
