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 lithistids, 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 1 that the 

 fibres of fossil Pharetrones are not original, but produced by fossilization, it is 

 difficult 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 Structuees oe Fossil Sponges. 



The skeleton of all known fossil Sponges is built up of small 2 mineral particles, 

 usually of microscopic dimensions, and of very varying forms, which are combined 



1 ' Die Pharetronen : Palaeontographica,' Bd. xxix, p. 283, 1883. 



3 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 antiqua 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. 



