328 Count de Bournon's Observations on a new 
acute isosceles triangle, the summit of which is truncated, and 
the sides of which are slightly bevelled. The bevel, however, is 
often so inconsiderable as to be scarcely perceptible. See 
Fi g- 5 - 
Each of the two sides which, in those pyramids, acquire the 
abovementioned extension of surface, at the expense of the two 
pthers, has constantly appeared to me to belong to one of the 
sides which forms the angle of 128°, but taken in an opposite 
direction, as is represented in Fig. 6, which is supposed to be 
the base of one of them. This base is perfectly similar, in the 
measure of its angles, to that of the hexaedral prism of Fig. 2 ; 
nevertheless, this hexaedral prism sometimes appears also to 
have the two planes, which have replaced the edges of 52 0 , more 
extended than the four others. 
In those crystals which are the most detached from each 
other, and exhibit a larger portion of their extent, it may be 
distinctly perceived, that the above pyramid is situated upon a 
hexaedral prism, of the same dimensions as the base of the 
pyramid ; (see Fig. 7 ;) but, as the angle formed by the junction 
of the sides of the prism with those of the pyramid is extremely 
obtuse, (its measure being 172 0 30',) the exact point of union 
of the prism with the pyramid cannot easily be distinguished. 
The summit of the pyramid is sometimes replaced by two 
trapezoidal planes, situated, when the pyramid is of a cuneiform 
shape, on the broadest sides. These planes meet together, at the 
summit, in a ridge of 110°, and form, with the sides of the py- 
ramid, on which they incline, an angle of 132 0 30'. See Fig. 8, 
At other times, this summit is replaced by two planes situated 
differently from the above, being placed on one of the edges 
contiguous to each of the two broad sides of the pyramid, and in 
opposition to each other. These planes, which are irregular 
