Koopmans, R. G. (1928). Researches on the flora of the coal-balls from the "Finefrau-Nebenbank" horizon in the province of Limburg (The Netherlands). N. V. Boek- en Handelsdrukkerij v/h Jos. Penners.
Lua i Stíl Chicago (17ú heag.)Koopmans, Reitze Gerben. Researches on the Flora of the Coal-balls from the "Finefrau-Nebenbank" Horizon in the Province of Limburg (The Netherlands). Heerlen: N. V. Boek- en Handelsdrukkerij v/h Jos. Penners, 1928.
Lua MLA (8ú heag.)Koopmans, Reitze Gerben. Researches on the Flora of the Coal-balls from the "Finefrau-Nebenbank" Horizon in the Province of Limburg (The Netherlands). N. V. Boek- en Handelsdrukkerij v/h Jos. Penners, 1928.
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