36 UNITED STATES GEOLOGICAL SURVEY— TERTIARY FLORA. 



of" plants of this order in former geological times, and has been able to 

 refer most of them to species allied or even identical with those of our 

 epoch. The Lichens mostly inhabit the bark of trees or closely adhere to the 

 surfaces of stones or rocks. Though hard they may appear, they are com- 

 posed of cellular tissue, easily destroyed by prolonged immersion, and it is 

 therefore extremely rare to find any species of this genus upon the bark of old 

 trees when their decomposition is advanced. They are also generally attached 

 to the epidermis, which is rarely preserved with the bark of decaying wood. 

 Though it may be, excepting the species described from the succin, very few 

 other Lichens have been recognized as yet in a fossil state. Goppert remarks 

 the presence of a Verrucaria and two Graphides in the Lignitic of Germany as 

 an extremely rare occurrence ; and Schimper says that he has been able to 

 find once only a fossil Lichen represented by a few specimens o\' Lecidea. It 

 was only after a careful study of the characters of the remains described 

 below that they were recognized as positively referable to Lichens. 



OPEGRAPHA, Ach., Nyl. 

 Opeg^rapha antiqiia, Lesqz. 



Plate I, Figs. 1-1 c. 



Opegrapha antiqua, Lesqx., Annual Report, 1872, p. 390. 



Perithecia one to four millimeters long ; linear or slightly enlarged in the middle ; pointed or 

 obtuse ; more generally united two, three, or lour in opposite directions ; sometimes flexuous or curved. 



The form of the small plants, or of their peritliecia, is distinctly repre- 

 sented upon the specimen, a piece of clay wherein the nuclei have either 

 been left imbedded or have deeply stamped their outlines. They are com- 

 paratively small, scarcely half a millimeter broad, either in a line or curved 

 in various ways ; sometimes short, oblong, enlarged in the middle, joined two, 

 three, or more by the points, and diverging star-like ; sometimes single, linear, 

 falcate or hooked, and obtuse at both ends. From their impressions, they 

 appear to have been upraised above the surface to which they were attached, 

 slightly grooved in the middle, with round borders. The inside part of the 

 cavities has generally in the middle, when tlie perithecia or nuclei are out, a 

 streak of black carbonaceous-like matter, as seen on the right of the 

 branch (enlarged part, fig. 1 a). The print of the nuclei is marked, figs. 

 1 6, 1 c, still more enlarged. As resulting from an impression, the inside 

 part should be convex, The borders of these small plants seem to have been 

 detached, or are too close to have allowed the penetration of clay between 

 them. The relation of this species is especially with Opegj-apha astrcza. 



