Stated as concisely as possible, the relationship is as follows: Each member of this chemical series of continuously varying composition can crystallise in two forms, which are the same for every member. The two varieties at one end of the series are called euxenite and priorite, at the other end polycrase and blomstrandine. Thus, whilst euxenite and priorite, at the one end, and polycrase and blomstrandine at the other, have the same compositions, euxenite and polycrase have the same crystalline form, whilst priorite and blomstrandine have the same second crystalline form. All four minerals have the same bright black appearance, and bright conchoidal fracture; they are all four isotropic, probably as a result of hydration. All are orthorhombic, but the measurements for euxenite and polycrase are different from those for blomstrandine and priorite. The two latter are not so widely distributed as the two former. Blomstrandine occurs at Hitterö, Arendal, and other localities in Norway; priorite is found in Swaziland, South Africa. The crystal system of the Polycrase-Euxenite series is orthorhombic, but Dana gives slightly different axial ratios for the two minerals. This, though Brögger gives the same values for both, is by no means incompatible with isomorphism, as a glance at the axial ratios for the minerals aragonite, strontianite, witherite, etc., of the series of the orthorhombic carbonates, will show. Brögger’s ratios for the two are _a_ : _b_ : _c_ = 0·3789 : 1 : 0·3527; Dana gives for polycrase 0·3462 : 1 : 0·3124, for euxenite 0·364 : 1 : 0·303. ~Euxenite.~ This species occurs usually in the massive form as a bright brownish-black mineral, of hardness 6¹⁄₂, and sp. gr. 4·6 to 5·0. The crystals are prismatic in habit; the common forms are the pinakoids _a_ {100} and _b_ {010}, the prism _m_ {110}, the unit pyramid _p_ {111}, and the dome {201}. Ramsay, Collie and Travers found no helium in it; Boltwood found uranium, radium and helium, and Strutt found in addition to these thorium. As early as 1879, Blomstrand had observed zirconium in euxenite. The mineral is infusible and with difficulty soluble in acids. It occurs in many localities in Scandinavia (Hitterö, Arendal, Brevig, etc.), in North Carolina, South Australia, etc. It was discovered by Scheerer at Jölster, in Norway, in 1839.