Searches for new quarks and leptons produced in Z-boson decay

We have searched for events with new-particle topologies in 390 hadronic Z decays with the Mark II detector at the SLAC Linear Collider. We place 95%-confidence-level lower limits of 40.7 GeV/${c}^{2}$ for the top-quark mass, 42.0 GeV/${c}^{2}$ for the mass of a fourth-generation charge -(1/3 quark, and 41.3 GeV/${c}^{2}$ for the mass of an unstable Dirac neutral lepton.