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j LOWER AND UPPER GREENSAND OF THE SOUTH OF ENGLAND. 435 
la recent tetractinellid sponges, the spicular elements of the skeleton are merely 
held in position by the sarcode, and the decomposition of this substance after the 
death of the animal would at once set the individual spicules free from each other. 
This structural character readily explains the complete manner in which these sponges 
have been broken up into their component spicules, for, though these are so abundant 
in the sponge-beds, I have not met with a single fragment of the skeleton with the 
spicules in their proper relative positions. Only under exceptionally favourable 
conditions of preservation could the entire body of the sponge be preserved in the 
fossil state. A few specimens have, however, been met with in the upper chalk of 
Yorkshire and Westphalia, but none, that I am aware of, in the greensand. 
The skeletal spicules of the lithistid sponges are more intimately intertwined and 
interlocked together than is the case with tetractinellid sponges, and it is therefore a 
matter for surprise that in these sponge-beds no fragments of the sponges themselves 
should be found. The spicules are thoroughly detached from each other, and I have 
not met with a single instance of even two spicules naturally united together. Entire 
lithistid sponges are, however, occasionally very abundant, in the upper greensand 
more particularly; but they appear in only a few limited localities, and their preser¬ 
vation seems to be owing to the infilling of the skeleton by chalcedonic silica before it 
was exposed to those influences which in the sponge-beds destroyed the union of the 
spicules with each other. 
It is a difficult matter to determine the extent to which the varied forms of these 
detached spicules may represent distinct species. There can be no doubt that 
numerous species are present in these sponge-beds ; but many of the characters, 
which in recent sponges determine the species and genus, are based on the arrange¬ 
ment of the spicules in the skeleton, the combination of various forms of spicules in 
the dermal layer, the characters of the canal system, and many other features, which, 
of course, are entirely unrecognisable in these detached spicules. The only classifica¬ 
tion which can be attempted in the circumstances must rest on the features of the 
isolated spicules, and where the form and proportions of these are sufficiently distinct 
from those of known recent and fossil sponges, they may reasonably be regarded as 
belonging to new species. These spicules, it is true, furnish us with only one charac¬ 
ter of the species, that is, the form of the skeletal elements ; but this, after all, is the 
most important feature. If the entire sponges should at any future time be dis¬ 
covered, of which, judging from present experience, there is but little probability, the 
provisional characters based on the spicules can then be supplemented by those derived 
from the entire skeleton. 
It is not my purpose in the present paper to discuss in detail the characters and 
relation of the spicular contents of these greensand sponge-beds, beyond what may be 
necessary to indicate the groups to which they may belong, and to furnish grounds 
for comparison with those of similar deposits elsewhere. In those cases in wdiich the 
spicules seem to me to indicate new species I have assigned a provisional name to 
