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specimen came from North America, being sent to Acharius by Muhlenburg. 

 It is the plant distributed under the name placorodia by Tuckerman in his Li- 

 chenes Americae SeptentrionaUs Exsiccati No. 71. Nylander's use of the name 

 for the isidiose plant is, therefore, incorrect and should not be continued. 



With few exceptions authors have correctly applied the name ambiguus 

 Wulfen. The original description (in Jacquin's Collectanea Botanica 4: 239. 

 1790.) states that the plant is " flavo-virescens . . tuberculis concoloribus 

 farinaceis" and the accompanying plate {T. 4, fig. 2.) shows the plant here re- 

 ferred to as " D." 



There remain to be considered three other names that have been connected 

 with the species under discussion. These are aleurites var. diffusa, hyperopia, 

 and ohsessa, all of Acharius (see synonymy given below for bibliographical ref- 

 erences). The type of Parmelia aleurites var. diffusa Ach. is made up of three 

 specimens, one from Sweden, one from Germany (collected by Schreber), and 

 one from France (collected by Dufour), all essentially the same, and closely 

 related to aleurites but smaller, with the lobes more irregular, and the isidia finer 

 and more scattered. The writer has collected material of this variety in Sweden, 

 not far from Stockholm and, therefore, in the region in which Acharius lived. 

 It is not uncommon in New England, and appears to be a form due to less favor- 

 able conditions in the habitat. It is, however, clearly recognizable and should 

 be retained as a valid variety. The type-specimen of ParmeUa hyperopia Ach. 

 from Switzerland is in poor condition but represents the species already named 

 Lichen diffusus by Weber. Parmelia obsessa Ach. (not Montague, which is a 

 Physcia) is represented by a specimen collected by Muhlenburg in North Amer- 

 ica. It is closest to aleurites, being copiously isidiose, but the under side is dark. 

 It may best be regarded as an aberrant form of Parmeliopsis aleurites. 



It will be convenient to sum up this discussion by giving a key to the North 

 American representatives of the genus Parmeliopsis, with the diagnoses and 

 synonymy of the species. 



A. Thallus without isidia or soredia. i. placorodia 



AA. Thallus with either isidia or soredia. 

 B. Thallus with isidia. 



C. Lobes regularly radiate and isidia densely crowded. 2. aleurites 



CC. Lobes more or less irregular and isidia fine and scattered. 



Note that all the species have the same habitat, namely, old fence-rails, 

 and the bark of conifers, especially of Pinus rigida. 



I. Parmeliopsis placorodia (Ach.) Nyl. 

 Parmelia Ach.. Syn. Lich. 196. 1814. 



Parmeliopsis Nyl. Flora 52: 445. 1869. as to nomenclature but not as 



2a. aleurites var. diffusa 



BB. Thallus with soredia. 



C. Thallus yellowish with sulphury soredia. 

 CC. Thallus gray, with whitish soredia. 



3. ambigua 

 4. diffusa 



to characters or plant! 



