PYRAMIDAL CLASS (

Hemimorphic)  . Here there is only one dyad axis in which two planes of symmetry intersect . The crystals are usually so placed that the dyad axis coincides with the vertical crystallographic axis, and the planes of symmetry are also vertical . The pyramid {hkl} has only four faces at one end or other of the crystal . The macro-prism and the brachy-prism of the last class are here represented by the macro-dome and brachy-dome respectively so called because of the resemblance of the pair of equally sloped faces to the roof of a house . The form {ool }. is a single plane at the top of the crystal, and is called a " pedion "; the parallel 'pedion (ooi} , if present at the lower end of the crystal, constitutes a different form . The prisms {hko} and the macro- and brachy-pinacoids are geometrically the same in this class as in the last . Crystals of this class are therefore differently developed at the two ends and are said to be " hemimorphic." Fig . 6o shows a crystal of the mineral hemimorphite (H2Zn2SiO3) which is a combination of the brachy-pinacoid {o101 and a prism, with the pedion (ooi), two brachy-domes and two macro-domes at the upper end, and a pyramid at the lower end . Examples of other substances belonging to this class are struvite (NH4MgPO4.6H20), bertrandite (H2Be4Si2O3), resorcin, and picric acid .