22 NEW YORK STATE MUSEUM 
pairs. Thus in figure 5 every plane of the second order prism is symmetri- 
cally disposed to its corresponding adjacent face on the right and left. 
A very limited number of dihexagonal prisms are recorded for calcite. 
These are 12-sided prisms the planes of which are parallel to the vertical 
axis and intersect all three of the basal axes at unequal distances. The 
alternate lateral edges are similar and the faces are symmetrical in adjacent 
pairs. The forms are of comparatively rare occurrence. 
Pyramids. Assuming the basal section of the prism of the second order 
shown in figure 5, suppose a series of double pyramids to be constructed 
+ 
“ 
eee 
having for common base the above mentioned basal section and with their 
vertexes lying on the vertical axis at definite proportions of its length. 
Such a series of pyramids is shown in figure 6 the proportions of the inter- 
cepts on the vertical axis being chosen to show some of the more frequent 
pyramids of the second order of calcite. The possible number of forms 
of this type is not limited save by the law of rational indexes. The symme- 
try of pyramids of the second order is that of the prism of the second order. 
In combination with forms of rhombohedral symmetry the faces of pyramids 
