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observation to confirm the opinion long entertained by many natu- 
ralists, that the tuberous forms of chalk flints and chert are due 
to organic bodies acting as nuclei, or centres of attraction, to the 
silex of which these tubereles are composed. Mr. Parkinson, in 
his interesting work on ‘Organic Remains of a Former World’ (1808, 
vol. ii. p. 87 et seg.), had noticed acicular spicula, which he found 
to be common to fossil sponges and fossil Aleyonia; and in pl. 7. 
fig. 8. of the same vol. he represents the magnified appearance of 
cruciform spines in a fossil Alcyonite resembling the Alcyonium 
cynodium of Linnzus, and quotes Donati as having described and 
delineated them before him. It has also long been known that a 
large proportion of the chalk flints in Wilts, Oxon, and Bucks, con- 
tain, within a grey external siliceous crust of variable thickness, a 
nucleus of semi-transparent flint, often of a purple tint, and exhibit- 
ing distinctly a congeries of tubes and net-work, nearly allied to 
modern Aleyonia; these Alcyonia were supposed to have acted as 
nuclei, or centres of attraction, which became first surrounded by 
the crust of grey flint, bearing no traces of organization, and subse- 
quently penetrated by a kind of red or purple chalcedony, taking the 
place of the particles of animal matter as they gradually decayed. 
This hypothesis has been modified by Mr. Bowerbank, whe has 
superadded the agency of parasitic sponges, which he supposes to 
have attached themselves to the aleyonic nuclei, and also to Echini 
and other shells, forming round these organic nuclei a covering or 
crust of sponge, which assumed, in its mode of growth, those irre- 
gularly tuberculated forms that are so common in, and are almost 
peculiar to, chalk flints. 
Having submitted to his microscope thin slices of chalk flints, in 
search of Foraminifera and Xanthidia, he observed, together with 
them, patches of brown reticulated tissue and spongiform spicula 
pervading the entire mass of the flints under examination ; this spongi- 
form structure was further pervaded by many tortuous cylindrical 
and minute canals of uniform diameter, which appeared to be the 
incurrent canals of the sponge, and by other orifices of greater dia- 
meter, resembling excurrent canals. He thinks that the mode in 
which the spicula, foraminifers, and other extraneous bodies are 
equably dispersed throughout the silex, shows that these bodies were 
entangled in the spongiform tissue in which their fossilization has 
taken place. 
With respect to the Echini and other shells, which are more or 
less filled with, or surrounded by grey flint, he supposes the para- 
sitic sponges to have grown both around and within the cavity of 
these shells, and in the case of Echini to have sometimes protruded 
outwards, sending forth branches through their orifices from the 
parasitic sponge within. He cites the parasitic habit of some modern 
sponges, which are found investing shells and other substances, in 
support of this hypothesis. 
In chalk flints from Wiltshire he found the spongiform structure 
aud spicula pervading the grey crust that enclosed many zoophytic | 
nyclei; but within these nuclei were neither spicula nor any of the 
