402 J. D. Dana on Lettering figures of Crystals. 
Po, 2Po , aPo, or 
2a, 1: 1:0, 2:1:a0, ©: 1:c), consists of planes par- 
allel to the shorter lateral axis, (which axis is therefore infinite 
In this system, the lateral axes are unequal, one being at right 
angles to the vertical, and the other inclined. e numbers re- 
ferring to the latter are distinguished by an accent, as in the ver- 
tical series 1-0’, 2-0’, 0-0’, (equivalent to P/ac, 2P’c, aP’c ). 
Those referring to the other lateral axis have no mark. 
In the ériclinic system, I vary a little from Naumann, approxi- 
mating more in plan to that for the monometric and trimetric 
pe oe a fp 
Wis 
. He 
ag. 
< ol 
fone nmere nn 
= 
- 
TRicLinic System. 
systems. The numbers referring to the longer horizontal axis 
have a long mark (— k 
~), those referring to the other a short mar 
(-); and the planes in a front view below the horizontal axes are 
marked with a minus. We have thus o-d and o-0 (Po and 
a Pa ) for the lateral planes of a prism parallel to the la‘eral 
axes. But as the axes are not only unequal, but cross at an 
oblique angle, and consequently have one olittise angle and one 
acute, another mark is. necessary ; and for the purpose of distine- 
tion, the planes pertaining to the acute angle in figure 5, have a0 
