418 FOSSIL FUXGI 



periods, and their offspring persisted to become the varied as- 

 semblage of species that constitute our present living fungi. 



In the ascending order the Tertiary period includes the Eocene, 

 Oligocene, and Miocene and grades into the Quaternary, includ- 

 ing the Pliocene and Recent Glacial. The Tertiary and Quater- 

 nary comprise the Cenozoic era. 



THE NATURE OF FOSSILIZED FUNGI 



It has been possible with a considerable degree of certitude to 

 relate fossil fungi with members of each of the classes employed 

 in classification of present-day forms. Some fungi, as is well 

 known, are extremely ephemeral; others, because of their corky, 

 leathery, or woody texture, can be kept indefinitely. Since fossils 

 both of ephemeral species, for example, phycomycetous forms, 

 and of resistant species, resembling Polyporus, occur, the paucity 

 of fossil fungi cannot be attributed solely to the constitution of 

 the fungi themselves. 



The fossilization of fungi is in no way different from that of 

 other plants. Ordinarily the term fossil implies that petrification, 

 a process in which living tissues are replaced by mineral matter, 

 has taken place. Sometimes in fossilization the replacement is 

 made with calcareous materials, as is the case with fossils found 

 in so-called "coal balls." These nodular concretions, sometimes 

 several inches in diameter, consist mainly of carbonates of calcium 

 and magnesium, together with oxides and sulphides of iron. 



Carbonaceous matter may also replace the original tissues in the 

 formation of fossils. 



Perhaps the most common kind of fossil is formed by incrusta- 

 tion with calcium carbonate. Sometimes leaves and stems, to- 

 gether with the fungi which inhabit them, leave impressions in 

 argillaceous or arenaceous shales or in travertine. These impres- 

 sions begin to form when the plant part is deposited in the siliceous 

 or calcareous matrix while it is still soft. Gradually the matrix 

 hardens and sets, and the impressions often portray the tissues 

 in great delicacy of detail. 



Sometimes fungi are found sealed up in masses of Baltic amber 

 and are thus preserved in a high degree of perfection. Baltic 

 amber, also called true amber, consists of hardened resinous secre- 

 tions that exuded from conifers and other trees during the Oligo- 



