264 PROCEEDINGS OF THE AMERICAN ACADEMY 



is found to include, or often conceal, a white or whitish true margin, 

 not containing gonidia, and appearing to belong to the proper exciple, 

 (but compare here the observations of Koerber, Parerg., p. 286,) being 

 traceable, in a vertical section, if I do not greatly mistake, downward 

 to the base of the apothecium ; which is, in that case, not lecideine (as 

 is assumed or asserted, by several writers, of the exciple of Trachylia), 

 but biatorine. And this biatorine structure appears, indeed, unques- 

 tionable in T. tigillaris, the apothecia of which (often conditioned by 

 the thallus similarly to those of the present), though finally and com- 

 monly black, and so described by all writers (Turner and Borrer 

 observing, however, Lich. Brit., p. 133, that the wall of the exciple, 

 seen in section, is "not black but grayish," to which reference will 

 again be made below), occur also in Lapland specimens collected by 

 Wahlenberg, and in others from this neighborhood, brownish, and even 

 whitish, both externally and in section. Nor does there seem to be 

 any difference of color, in a section of the palest of these apothecia 

 between the base (hypothecium) and the margin ; but, as the exciple 

 blackens above, a very thin continuous line of the same color is ob- 

 servable also below, and, within this, the more conspicuous white layer, 

 extending also upwards, to which we have already referred. And this 

 double hypothecium, variously modified, appears in fact characteristical 

 of Trachylia, as a genus. It is seen in T. Calif ornica (showing in 

 section a white hypothecial layer, and below that a thicker blackish- 

 brown one, disappearing, or at least the color, below the excipular 

 margin), and is marked in T. tym-panella (where the white layer is 

 readily traceable into the powdery inner border characterizing that and 

 other species, as indicated in Acharius's figure, Lichenogr. t. 3, f. 1, 

 while the much-thickened, dark-brown or blackish, inferior layer forms 

 the outer, black one), with which last T. lecideina, Nyl, (Lich. Par. n. 

 18), T. stigonella (Moug. & Nestl. Cr. Vog. n. 858), and T. lucida 

 (Th. Fr.) sufficiently agree ; and the structure is also the same in T. 

 leucampyx, Tuckerm. (Wright. Lich. Cub. n. 21), a species especially 

 remarkable for the prominence of the white margin, which finally over- 

 lays and conceals the proper black one, as well as in T.javanica, (M. 

 &, V. d. B.) Nyl. (Lich. Cub. n. 22), in which the white layer is re- 

 duced to a mere film, and the black exciple has its most extraordinary 

 development. The view of the structure of the exciple of Acolium 

 ocellatum, Koerb. {Trachylia, Plot.), otherwise unknown to me, taken 

 in the learned author's exhaustive description (Parerg. pp. 285, 286), 



