36 PALuEONTOLOGICAL BOTANY. 



thigh. Schimper regards the Psilophyton of Dawson (Plate 

 IV. Fig. 5) as allied to Pilularia, one of the Rhizocarps (Fig. 22), 

 and Carruthers ]3laces it among the true Lycopodiacea3. 



The Carboniferous period is one of the most important 

 as regards fossil plants. The vegetable forms are numerous, 

 and have a great similarity throughout the whole system, 

 whether exhibited in the Old or the New World. The 

 important substance called Coal owes its origin to the 

 plants of this epoch. It has been subjected to great 

 pressure and long-continued metamorphic action, and hence 

 the appearance of the plants has been much altered. It 

 is difficult to give a definition of Coal. The varieties of it 

 are numerous. There is a gradual transition from Anthracite 

 to Household and Parrot Coal ; and the limit between Coal 

 and what is called bituminous shale is by no means distinct. 

 Coal may be said to be chemically-altered vegetable matter 

 interstratified with the rocks, and capable of being used as 

 fuel. On examining thin sections of coal under the micro- 

 scope, we can detect vegetable tissues both of a cellular and 

 vascular nature. In AVigan cannel coal, vegetable structure 

 is seen throughout the whole mass. Such is likewise the 

 case with other cannel, parrot, and gas coals. In common 

 household coal, also, evident traces of organic tissue have 

 been observed. In some kinds of coal punctated woody tissue 

 (Plate III. Fig. 5) has been detected, in others scalariform 

 tissue (Plate III. Fig. 6), as well as cells of different kinds. 

 Sporangia are also frequently found in the'substance of coal, 

 as shown by Mr. Daw in that from Fordel (Plate III. Figs. 

 1 to 3) ; and some beds, like the Better bed of Bradford, are 

 composed almost entirely of these sporangia imbedded in 

 their shed microspores, as has been recently shown by Huxley. 

 The structure of coal in different beds, and in different parts 

 of the same bed, seems to vary according to the nature of 

 the plants by which it has been formed, as well as to the 

 metamorj)hic action which it has undergone. Hence the 



