whose
base lines are all in one plane. This form rarely occurs except in
combinations. (3.) Tetragonal sphenoids (fig. 28), bounded by four
isosceles triangles, are the hemihedral forms of the first variety of
tetragonal pyramids. (4.) The tetragonal scalenohedron (fig. 29),
bounded by eight scalene triangles, whose bases rise and fall in a
zig-zag line, is the hemihedral form of the ditetragonal pyramid. The
latter two forms are rare.
Open forms.—Tetragonal
prisms (fig. 30) bounded by four planes parallel to the principal axis;
ditetragonal prisms by eight similar planes. In these prisms the
principal axis is supposed to be prolonged infinitely, or to be
unbounded. Where it is very short and the lateral axes
