128 
Wiltshire fossil in the frontispiece, different in its structure from all 
the rest, is formed of a spongeous substance, which is pervaded and 
embraced by ramifying tubuli or fibres. 
Thus, if the figure of the fossil be assumed as the leading character 
of the species, substances, differing materially in their structure, will 
be classed together in the same species ; and, on the other hand, if the 
species be formed on the external structure, we shall have, under the 
same species, substances differing as widely in their forms as the fossils 
at Plate XL Fig. 6 and 7- 
LETTER XIV. 
ALCYONITES INCLOSED IN FLINTS FOUND IN WILTSHIRE, OX- 
FORDSHIRE, BUCKINGHAMSHIRE, AND HERTFORDSHIRE OTHER 
SPECIMENS NOTICED. 
It is to my truly learned and respectable friend, the Rev. Mr. Towns- 
end, that I owe the favour of having my attention first called to the 
fossils, the investigation of which I shall next attempt. That gentle- 
man not only obliged me with a sight of his own beautiful drawings, 
but also presented me with a specimen of hydrophanous flint, which, 
with many other more perfect specimens, he had himself found in the 
Vale of Pewsey. These he shewed me were evidently full of changed 
organized matter, belonging to some unknown species of zoophyte : of 
which he pointed out to me not only their internal parts, but even their 
cortical investments. 
These flints were of an oblong, and somewhat of a rounded form ; they 
seeming to differ from the common black flint, in having one of their 
