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HERRE, A. W. T. C. 1946. The Parmelias of California. Contrib. Dudley Herb. 3: 313-350. MAAS GEESTERANUS, R. A. 1947. Revision of the lichens of the Netherlands. I. Parmeliaceae. Blumea 6: i-viii + 1-199. SANTESSON, R. 1944. Contributions to the lichen flora of South America. Ark. Bot. 31A(7) : 1-28. ZAHLBRUCKNER, A. 1913. Schedae ad Kryptogamas exsiccatas editae a Museo Palatino Vindobonensi. Centuria XXI. Ann. K. K. Naturhist. Hofmus. [Wien] 27: 253-280.
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John W. Thomson (1955) studied this question.
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