64 BRITISH FOSSIL SPONGES. 
Chalk. The manner in which these detached spicules are dispersed in the matrix, 
and their perfect forms, show that this disintegration took place shortly after the 
death of the Sponge, and before it was covered with sediment, and also that they 
have not been transported far from the locality in which the Sponges existed. 
The dispersion of the spicules, when they are not united together into a connected 
skeleton, is very strikingly shown in the case of those forming the dermal layer of 
lithistid Sponges, which, unlike those of the body of the Sponge, are free and 
merely held in position by the penetration of their pin-like shafts into the Sponge. 
Though there is every reason to suppose that fossil lthistids, like recent ones, 
were uniformly provided with a dermal layer of differentiated spicules, this is very 
rarely now retained, but the detached spicules themselves are not infrequently met 
with dispersed in the matrix. 
In the dictyonine group of fossil hexactinellid Sponges, the spicules are com- 
pletely amalgamated together, and consequently these Sponges retain their entire 
form even though their walls are frequently of a thin and delicate character. In 
the lyssakine group, on the other hand, in which the spicules are free from each 
other, examples of entire Sponges are very rare, but the detached spicules are very 
abundant in the Lower Carboniferous of Ayrshire and Yorkshire, and also in the 
Upper Chalk. 
Fossil calcisponges as a rule retain their entire forms, and detached spicules of 
these Sponges are rarely met with. This preservation of the entire form is the 
more surprising since the component spicules of their skeletal fibres are not organi- 
cally attached together, and their habitat in shallower water would have exposed 
them to more disturbed conditions. The spicules in these fibres appear, however, 
to have been very closely and intimately arranged, and to this may be due the fact 
that they have not been disintegrated. On the theory of Dunikowski' that the 
fibres of fossil Pharetrones are not original, but produced by fossilization, it is 
dificult to understand how they could have retained their entire forms, since in all 
other cases, as we have seen, the effects of fossilization are rather to break up and 
disintegrate the spicular structures than to consolidate them. 
SKELETAL Structures oF Fossit SpPonaceEs. 
The skeleton of all known fossil Sponges is built up of small’ mineral particles, 
usually of microscopic dimensions, and of very varying forms, which are combined 
? “Die Pharetronen: Palaeontographica,’ Bd. xxix, p. 288, 1883. 
* Sponges with skeletons of horny fibres have not yet been definitely shown to exist as fossil. 
Though various forms have by some authors been referred to horny Sponges, there is no satisfactory 
evidence that they belong to this group. The Dysidea antigua of Carter, ‘Annals and Mag. Nat. 
Hist.,’ 1878, vol. i, p. 139, described as a horny Sponge, is a siliceous monactinellid, which had been 
previously named by Young and Young, Haplistion Armstrongi, ‘ Annals, &c.,’ 1877, vol. xx, p. 428. 
