422 



larly disposed in the wider and almost oval interstices, between the 

 nearly semicircular receding waves of undulating lines. This form, 

 so difficult to describe correctly, is well represented in the third 

 number of Mr. Martin's Derbyshire Petrifactions. It is also to be 

 seen in Volckman's Silesia Subterranea. 



Plate III. Tab. IV. Fig. 9. Mr. Da Costa's sixth figure, you will 

 find is very successfully represented in PI. V. No. 8. A careful exa- 

 mination of the specimen from which this figure was drawn, furnishes 

 some information respecting the nature and the formation of these 

 fossils. This specimen, evidently a piece of shale not quite half an 

 inch in thickness, bears a similar impression on each side ; and having 

 been purposely placed in water, and allowed to remain there about 

 two hours, became very soft and friable : the internal part becoming 

 resolved into a pulpy mass, which had evidently been formed of a 

 confused heap of grass, and other vegetable matters, intermixed 

 with dark argillaceous earth. The external part appeared to have 

 been the epidermis of some species of cactus : the internal suc- 

 culent part having been washed away, and its place supplied by 

 these foreign matters, which have become enclosed in it, and have 

 prevented its sides from having quite collapsed together. Thus 

 may we account for the different degrees of thickness which these 

 fossils possess ; this depending partly on their original form, and 

 partly at the distance at which the outer coat has been kept during 

 the resolution of their internal substance. The form of these fossils 

 may depend also on another circumstance. The plant being buried 

 in a soft margaceous mass, its external coat may remain attached 

 to the inside of the mould, which it had formed, whilst the cavity, 

 left by the resolution of its internal succulent part, would gradually 

 be filled up by the introduction of earthy matter. The cortical 

 part, thus enclosed in earthy matter, would undergo the bituminous 

 change; and the bitumen, thus filling up the impressions in the 

 mould, which the external surface had formed, would present the 



