440 Mr. Greville on the 
This decrease necessarily produces a single pyramid, as 
abovementioned ; nevertheless, there are instances of crystals of 
Corundum, belonging to the variety where the terminal planes 
make, with the planes of the pyramid, a solid angle of about 
ioo°, in which, two pyramids of the same dimensions, having 
their summit replaced by a small hexagonal plane, are placed 
base to base. 
I have also observed, among the crystals of the obtuse va- 
riety abovementioned, in Mr. Greville’ s collection, an instance 
of the decrease taking place by several rows, on one three-sided 
pyramid of the primitive rhomboid, and by single rows on the 
other. Consequently, the crystal is a short regular hexaedral 
prism, terminating on one end only by an hexaedral pyramid ; 
the planes of which, as well as of the prism, are alternately 
broad and narrow, and almost perfect ; its apex being replaced 
by a very small plane. 
I shall conclude, by mentioning a variety of Corundum, de- 
scribed by the Abbe Hauy, in the Journal des Aimes, No. 28; in 
which, the edges of the terminal planes of the hexaedral prism 
are replaced by planes which form an angle of ii6 0 3i A , with 
the terminal planes ; but, in the numerous collection of Mr. 
Greville, I have not seen this variety. One crystal had an 
appearance of such planes ; but, on examination, it was clearly 
accidental. The authority of the Abbe Hauy, in crystallogra- 
phy, is so great, that the existence of such modification ought 
not to be denied, without further examination; though! cannot 
in this instance adopt it : he derives this variety, which he 
calls subpyramidal, from a decrease of three rows of molecules, 
at the angles of the base of the two pyramids of the primitive 
