478 DOME-SHAPED STEM. 
In many of the strata that accompany the coal, 
fragments of these plants occur in vast abun- 
dance ; they have been long noticed in the sand- 
stone called Gannister and Croivsioiie, in the 
Yorkshire and Derbyshire coal fields, and have 
been incorrectly considered to be fragments of 
the stems of Cacti. 
The discovery of the dome-shaped centres 
above described, and the length and forms of the 
leaves and branches render it highly probable 
that the Stigmariae were aquatic plants, trailing 
in swamps, or floating in still and shallow lakes, 
like the modern Stratiotes and Isoetes. From 
such situations they may have been drifted by 
the same inundations, that transported the Ferns 
and other land vegetables, with which they are 
associated in the coal formation. The form of 
the trunk and branches shews that they could 
not have risen upwards into the air ; they must 
therefore either have trailed on the ground, or 
have floated in water.* The Stigmaria was pro- 
bably dicotyledonous, and its internal structure 
seems to have borne some analogies to that of 
the Euphorbiacese. 
* The place and form of the leaves, supposing them to have 
grown on all sides of branches suspended horizontally in water, 
would have been but little changed by being drifted into, and 
sinking to the bottom of, an estuary or sea, and there becoming 
surrounded by sediments of mud or sand. This hypothesis seems 
supported by the observations made at Jarrow, that the extre- 
mities of the branches descend from the dome towards the adja- 
cent bed of coal. 
