Summary: Oscillation, Laue, and rotation photographs show that descloizite has an orthorhombic unit cell with edges a 6·05, b 9·39, c 7·56 Å., and space group Vh16. The unit cell contains 4PbZn(OH)(VO4). Powder photographs of descloizite, cuprodescloizite, mottramite, psittacinite, chileite, eusynchite, and dechenite, from the type localities are identical with each other. New chemical analyses and determinations of the water content at various temperatures, together with the X-ray work, show that all these minerals may be represented by the general formula Pb(Cu,Zn)(OH)(VO4). Thin incrustations of minute black crystals on sandstone from Harmer Hill, near Shrewsbury, are identical with the original mottramite from Mottram St. Andrew, Cheshire. Mottramite is retained as the most suitable name for all descloizites containing more than 10% CuO.