Additional Information:

In Dawson's opinion this strata was deposited as follows:

"On this peaty soil grew the trees represented by the stump of mineral  charcoal
  mentioned  above," 
(not shown)  "and  which  were  probably  coniferous.  This
  tree,  being  about 1 foot  in  diameter,  must  have  required  about  fifty  years
  for its growth... It was then killed, perhaps by the inundation of the bog. During
 
(its) decay... Sigillariae, d, grew...  to the diameter of  two  feet, when  they  were
 overwhelmed by  sediment,  which  buried  their  roots  to  a  depth  of  about 18
 inches.  At  this  level  Calamites,  a,  and  another  Sigillaria
(also  not  shown)
 "began  to  grow,  the  former  attaining  a  diameter of  4  inches,  the  latter  a
 diameter of about a foot
... These plants were in...  turn  imbedded  in  somewhat
 coarser sediment, but so gradually that the trees with Stigmarian roots, c,  grew
 at two higher levels before the accumulation of mud and sand attained a depth
 of  9  feet,   at  which  depth  the  original  large  Sigillariae,   that  had   grown
 immediately over the coal, were broken off, and their hollow trunks filled  with
 sand
..."
1   Emphasis Added

From this we note that there were three other tree stumps in this section that were not included in the above drawing.  One of these was "rooted" in coal at (or below) the level of the 9 foot tree pictured; this was the stump that contained the remains of a lizard, a land-shell, and  "a mass of vegetable fragments, such as Calamites, Poacites, and a Lepidostrobus, evidently introduced before the sedimentary matter..." p. 22.  The other stump was "rooted"  18 inches higher at the same level as the lower Calamites (a).  It is worth noting that this higher stump had Stigmaria  roots attached.  Regarding this Dawson says:

"We...  shall confine our present remarks to a few  strata,  not  more  than 10  feet
  thick, which enclose certain vertical trees;--one of these trees  having Stigmaria
  roots...
2  Emphasis added

Regarding these attached roots we are told:

"Thus exposed, the sandstone No. 5 (section 2 in drawing above) shows... a stump 1
  foot in  height and  1 foot  in  diameter,  having  its  surface- markings destroyed
  by the sea,  but retaining two  distinctly  marked  Stigmaria  roots,  one  of  them
  bifurcating at the distance of 2 feet from the stump
."
3
 Emphasis added

With Regard to the plants, stems and roots we are told:

 "In the cliff are seen, in the same bed, Calamites, rooted and erect, a plant stem,
   somewhat inclined, but  apparently  having roots attached, and a large vertical
   tree, 2 feet in diameter at base, and  fluted  irregularly, but  without  leaf-scars.
   This tree springs from  the coal No. 6 and  extends to the top of  No. 5,  but  the
   Calamites and stems of some unknown plant are rooted at a higher  level,  and
   there  are  Stigmaria roots, apparently in situ,  at  three  distinct  levels  in the
   sandstone"
  3    Emphasis added

Note: In the above quotes, Coal No. 6 is the same as Coal No. 3 (in Dawson's drawing) and Bed No. 5 is the same as Bed No. 2.  The confusion arises because Dawson re-numbered the sections in his next paper that appeared in the next issue (Vol. 10) of this Journal.

Regarding the tree with the reptilian, shell, and plant remains we are also given the following details:

"In tracing this bed a little beyond the large tree represented in the section.., we
 found on the beach another fossil tree, 22 inches in  diameter, which had  fallen
 from the bed No. 5, and had no doubt occupied a  position similar to that of  the
 large tree still standing.  It was a cylindrical cast in  sandstone,  having  on  the
 surface flutings like those of Sigillaria, but without leaf-scars.  In the lower part
 of the cast, the sandstone contained a large quantity of vegetable  fragments
, as
 above  mentioned  (p. 59),  principally  pieces  of  carbonized  wood,   leaves  of
 Naeggerathia or Poacites?
, and stems of Calamites.  With  these...we found  the
 bones, jaws, teeth, &c. before alluded to, all  distinctly  within the lower part of
 the cast,
and scattered among the vegetable fragments... as if either  washed in
 separate pieces, or... mixed  with  the  woody  matter..."
31   Emphasis added

References:

1.   Dawson, 1854, Quart. Jour. Geol. Soc. London, Vol. 10, p. 21; 
2.   Lyell, Sir Charles, and Dawson, Sir John W., 1853, Quart. Jour. Geol. Soc. London, Vol. 9, p. 58-59.
3.   ibid. ref. 2
, p. 62..