* 62 BRITISH FOSSIL SPONGES. 
In the calcisponges from the Lower Greensand of Faringdon, Berkshire, the fibres 
are of a greyish tint, and the crystallization has largely obliterated the spicular 
structure, and in many instances the fibres are invested with an outer coating of 
calcite crystals. In the Jurassic calcisponges the outer form of the Sponge and 
the fibres are well preserved, but their minute spicular components have been, to 
a large extent, destroyed by the secondary crystallization of the calcite, and only 
occasionally are the larger spicules faintly shown. 
It is worthy of notice that, in striking contrast to many siliceous Sponges, in 
which the skeleton has been preserved in a negative form, as hollow moulds in an 
enclosing matrix, no undoubted calcisponge has, to my knowledge, been met with 
in this condition, but in all cases the fibres are present, either as calcite, or, more 
rarely, replaced by crystalline silica. 
Caleisponges replaced by Silica.—This replacement is of infrequent occurrence, 
and it usually takes place in strata in which Corals and other calcareous organisms 
as well as the Sponges have become siliceous. Calcisponges thus silicified can be 
obtained quite free from matrix by treatment with acid; their fibres are then of 
a snowy-white tint, with rough and uneven surfaces, and all traces of the spicular 
structure have disappeared. 
An exceptional mode of preservation is shown in some examples of Pharetro- 
spongia from the so-called Cambridge Greensand and from the Upper Chalk of 
Kent. The Sponges are usually calcareous throughout, but in some instances a 
partial replacement of the fibres by silica has taken place. When treated with 
acid the interior portion of the fibres, being calcareous, is dissolved, but a thin 
exterior pellicle of silica remains, in which the form of the minute acerate spicules 
can be seen. In the Upper Chalk examples, those which are embedded in the 
chalky matrix have their fibres entirely calcareous, and they are completely dissolved 
in acid, whilst in the specimens embedded in flint the outer layer of the fibres is 
replaced so as to form a thin pellicle of silica, the same as in the Cambridge Green- 
sand specimens. This different mineral condition of the fibres, according as they 
are enclosed in chalk or flint, may be observed not infrequently in the same speci- 
men, part of which is in the flint, and has the silicified surface-pellicle enveloping 
the fibre, whilst another portion is in the chalk, and the fibres are throughout 
calcareous. ‘There can be no doubt that the thin surface-film of silica on the fibres 
of the flint-enclosed examples of Pharetrospongia is owing to the replacement of 
the original calcite by the silica, such as usually takes place in calcareous organisms 
enclosed in flints. It has been contended that this genus was originally siliceous, 
and that the calcareous fibres result from the replacement of the original silica by 
calcite. That, however, the forms are really calcisponges is further shown by the 
fact that no other undoubted siliceous Sponge in the Upper Chalk presents the 
same mineral structure and appearance as these examples of Pharetrospongia. 
