422 
disposed in the wider and almost oval interstices, between the nearly 
semicircular receding waves of undulating lines. This form, so dif- 
ficult 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 on 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 exact figure of 
