60 THE GEOLOGICAL HISTORY OF PLANTS. 



or antberidia, are very delicate and destitute of the dense 

 outer wall of the macrospores. Hence such parts are 

 little likely to have been preserved in a fossil state ; and 

 in the Erian shales, if present, they probably appear 

 merely as flocculent carbonaceous matter not distinctly 

 marked, or as minute gi-anules not well defined, of which 

 there are great quantities in some of the shales. 



The vegetation appertaining to the Sporangites has 

 not been distinctly recognised, I have, however, found 

 in one of the Brazilian specimens two sporocarps attached 

 to what seems a fragment of a cellular frond, and numer- 

 ous specimens of the supposed Algae, named Spirophyton, 

 are found in the shales, but there is no evidence of any 

 connection of this plant with the Protosalvinia. 



Modern Ehizocarps present considerable differences as 

 to their vegetative parts. Some, like Pilularia, have 

 simple linear leaves ; others, like Marsilea, have leaves in 

 whorls, and cuneate in form ; while others, like Azolla 

 and Salvinia, have frondose leaves, more or less pinnate 

 in their arrangement. If we inquire as to fossils repre- 

 senting these forms of vegetation, we shall find that some 

 of the plants to be noticed in the immediate sequel may 

 have been nearly allied to the Ehizocarps. In the mean 

 time I may state that I have proposed the generic name 

 Protosalvinia for these curious macrospores and their 

 coverings, and have described in the paper in the " Bul- 

 letin of the Chicago Academy of Sciences," already 

 quoted, five species which may be referred to this genus. 



These facts lead to inquiries as to the origin of the 

 bituminous matter which naturally escapes from the 

 rocks of the earth as petroleum and inflammable gas, or 

 which may be obtained from certain shales in these forms 

 by distillation. These products are compounds of carbon 

 and hydrogen, and may be procured from recent vegetable 

 substances by destructive distillation. Some vegetable 

 matters, also, are much richer in carbon and hydrogen 



