Biostratigrafi︠i︡a nizhneĭ chasti ordovika Severo-Vostoka SSSR i biogeografi︠i︡a kon︠t︡sa rannego ordovika - Keller, Boris Maksimovich; Chugaeva, M. N. (Mari︠i︡a Nikolaevna) (Trudy (Geologicheskiĭ institut (Akademi︠i︡a nauk SSSR)), 1973)