o32 
THE GEOLOGIST. 
are glaciers in latitude 50 clegs, north. The Snowdon group is at 
least three degi*ees further north than this, and, if instead of being 
part of an island, that district, at a higher elevation, were pai-t of a 
broad continent spreading east and west, we should have in its peaks 
and valleys all the conditions needful for the formation of large 
glaciers ; and the same may be said of other mountain regions of 
the British Islands." 
I now send you these speculations for what they are worth, 
although in the long run I think it will appear that the wdde-spread- 
iug, long-continued, and most intense cold of the glacial period was 
due to some unexplained cause far more general than any mere 
changes in physical geogi'aphy. 
tiRITISH ASSOCIATION MEETING. 
ON A NEW POINT IN THE STRUCTURE OF SIGILLARIA, WITH SOME 
REMARKS ON THE BIVALVES OF THE COAL. 
By J. W. Saltek, Esq., F.G.S. 
Among the veiy fine collections of fossil-plants in the Manciiester Museum, 
arc some specimens of SigUlaria well worthy attention, and which show, as I 
believe, a new point of structure, bearing on their aqnatic habits. 
They belong to that section of the genus which is distinguished by the leaf- 
scars, being arranged close together in the vertical rows, not at a distance apart, 
as inmost of the species. So close, indeed, that they press on one another, and 
compel each other to take an hexagonal form. 
At certain points alon^ the trunk, ucav lines of scars are interpolated, so as 
to make the number of ridges greater (and at the same time the individual 
ridges narrower) in the younger portions of the tree. In Favidaria these 
intercalations very much at particular spots, forming a sort of varix, or 
node, not very obscurely marked. At these points, too, in cerfain species, the 
stem is swelled, the spaces between the ridges deepened — the ridges themselves 
narrower and more prominent, and altogether angular in form. Brongniart's 
artist has badly represented such a varicose swelling in his figure of SigUlaria 
lie.rafiona. 
The species in which I have observed this characteristic are : — the YavuJaria 
teueJIata, from Tonge, Bolton ; F. nodow^ from Oldliani, and a species from 
Glamorganshire ; F. hexagona, from ]Manchester coal-field. 
In a ])ei-fectly preserved stem of F. tessellata, fi-om the roof of tlie "four- 
foot" coal, the intervals between the ridges are occupied by rows of circular 
scars, not hexagonal, nor purse-sbaped, as the leaf-scars between them are, but 
round. Nor with the cliaract eristic flattened surface and double imprint from 
which the vascular bundles of the leaf arise, but deeply indented, and with a 
minute protuberance in the centre, which structure is characteriseic of the 
rootlets of the underground portion of the stem (Stif/jnaria). 
The look of these small interpolated sears is so dilferent from those of t 
leaf-rows (they are clearly not interpolated ridges, for they die out at each 
