domes.
The inclined prisms are often designated clino-domes, the term prism
being restricted to the vertical forms. Orthopinacoids and
clinopinacoids are also distinguished from their position in relation
to the axes.
The monoclinohedric pyramids (fig. 58) are bounded by
eight
scalene triangles of two kinds, four and four only being similar.
Their lateral edges lie all in one plane, and the similar triangles are
placed in pairs on the clino-diagonal polar edges. The two pairs in the
acute angle between the orthodiagonal and basal section are designated
the positive hemipyra-mid; while the two pairs in the obtuse angles of
the same sections form together the negative hemipyramid. But as these
hemipyraraids are wholly independent of each other, they are rarely
observed combined. More frequently each occurs alone, and then forms a
prism-like figure, with faces parallel to the polar edges, and open at
the extremities. Hence, like all prisms, they can only appear in
combination with other forms. The vertical prisms are bounded by four
equal faces parallel to the principal axis, and the cross section is a
rhomb; the clinodomes have a similar form and section; while the
horizontal prisms or domes have unequal faces, and their section is a
rhomboid.
The
mode of derivation of these forms closely resembles that of the rhombic
series. A complete pyramid is assumed as the fundamental form, and
designated ± P, in order to express the two portions of which it
consists. Its dimensions are given when the proportion of its axes a;b;c
