DIMETRIC, OR TETRAGONAL SYSTEM. 31 
being exactly alike in inclination to the basal plane as well as 
alike in lustre. 
There are two distinct square prisms. In one (fig. 10) the 
10. A, 

axes connect the centres of the lateral faces. In the other 
(tig. 12) they connect the centres of the lateral edges. In fig. 
11 the two prisms are combined; the figure shows that the 
planes of one truncate the lateral edges of the other, the inter- 
facial angle between adjoining planes being 135°. Figs. 2, 3, 
4,7, are of others having planes of both prisms. In fig. 13 one 
pr ism 1s represented within the other. 
Fig. 14 represents an eight-sided prism, and fig. 15 a combi- 
nation of a square prism G. t)} with an eight- sided prism (¢-2). 

Another example of this is shown in fig. 4, and also in fig. 9, 
the planes 7-2 in one, and 7-3 in the other. 
The basal plane in these prisms is an independent plane, be- 
cause the vertical axis is not equal to the lateral, and hence it 
almost always differs in lustre and smoothness from the lateral. 
Like the square prisms, the square octahedrons are in two 
series, one set (fig. 16) having the lateral or basal edges parallel 
to the lateral axes, and these axes connecting the centres of 
opposite basal edges, and the other (fig. 17) having them diago- 
nal to the axes, these axes connecting the apices of the opposite 
