40 AN INTRODUCTION TO THE STUDY OF FOSSILS 



hotter springs the masses become so densely gelatinous, or so 

 thickly encrusted with silica, as not to be easily recognized. 



Doubtful algae. — It has been shown that many fossils which 

 were formerly described as algae are not plants. Some are 

 the molds of burrows or tracks of animals and some are of inor- 

 ganic agency, such as Oldhamia from the Cambrian of Ireland, 

 formerly considered the oldest of all vegetation and now ex- 

 plained as merely the wrinkhng of the slate due to pressure. 

 The " Cockstail alga " (Taonurus cauda-galli) which occurs in 

 great abundance in the " Cauda galli grits " (Esopus) of the 

 Middle Devonian of New York, and Arthrophycus harlani, 

 abundant in the Medina sandstone of the Silurian, are at 

 present generally regarded as the burrows of sedentary chaeto- 

 pod worms. 



SUB-DIVISION E, FUNGI 



These plants are especially distinguished by the absence of 

 chlorophyl. They are thus unable to manufacture starch and 

 sugar from the soil and air and must live on that already manu- 

 factured, — that is, upon other organic matter. Accordingly 

 they live either as saprophytes upon decaying organic matter 

 or as parasites upon living organisms. 



Instead of possessing the more complex structure of chloro- 

 phyl-bearing plants, a fungus consists essentially only of a branch- 

 ing mass of threads called the mycelium. These threads pene- 

 trate the cell walls of their ''host," — plant or animal, — and live 

 upon its substance. Toadstools and mushrooms, molds, mil- 

 dews and yeast are common examples. 



As would be expected from their structure, fungi are rarely 

 preserved as fossils. Mycelium threads of a fungus have been 

 detected under the bark of Sigillaria from the Pennsylvanian 

 coal-beds, and there is evidence that even as far back as the Si- 

 lurian, fungi preyed upon "shell-fish," since certain brachiopod 

 shells are found more or less perforated by fine tubules which in 

 some cases end in spherical sweUings. These borings (Fig. lo) 



