Tetragonal bipyramid of the first order (figs. 42 and 43). This is
bounded by eight equal isosceles triangles. Equal lengths are
intercepted on the two horizontal axes, and the indices are {111},
{221}, {112}, &c., or in general {hhl}. The parametral plane with the
intercepts a : a : c is a face of the bipyramid {111}.
[Illustration: FIG. 44.]
[Illustration: FIG. 45.