Measurements of R, sphericity and thrust are presented for c.m. energies between 12 and 31.6 GeV. A possible contribution of att-continuum can be ruled out for c.m. energies between 16 and 31 GeV. If quarks occur in weak isospin doublets, a sixth quark t, with charge 2/3 is required to complement the known u, d, s, c, and b quarks. Predictions for the mass of the t quark cover a wide range of values [-1] ; the majority are within the reach of present PETRA energies, namely 10 to 15 GeV. The presence of a new