54. BRITISH FOSSIL SPONGES. 
are frequently of considerable thickness, and the interspaces between the skeletal 
fibres are comparatively large, no distinctive canals are present. In other calcisponges 
a system of branching canals extends from the interior of the wall, either to the 
cloacal cavity or to the general upper surface of the Sponge, terminating in single 
vents or in small groups of vents. In some forms also open canals radiate from 
the margins of the cloacal cavity down the sides of the Sponge. Examples of this 
structure are shown in Sestrostomella, Stellispongia, and Lymnorea. Inthe genera 
Hlasmostoma and Rhaphidonema, one surface of the sponge-wall is covered with a 
minutely perforate dermal layer, whilst the opposite surface has larger scattered 
vents, which are connected with branching canals. In Tremacystia the wall, 
consisting of but a single layer of spicular fibre, is perforated by circular apertures 
which lead into hollow chambers, and these latter are connected together by a 
cribriform axial tube, which opens at the summit of the Sponge and probably 
represents the cloaca. 
CaremicaL ConstituTION AND MoprE or Preservation or Fossiz Sponanas. 
Only those Sponges are definitely known as fossil in which the skeleton was 
originally either of siliceous or calcareous composition. Sponges with keratose or 
horny skeletons have not with certainty been shown to exist in the fossil state. 
The changes which have taken place in the mineral structure of both siliceous 
and calcareous Sponges during fossilization, have, in many cases, so completely 
altered their original nature that it requires careful study and considerable 
experience to determine the groups to which they belong. It is owing to these 
changes not having been properly understood that so much misconception 
has arisen respecting the real nature of these organisms, and the sup- 
posed radical differences between them and existing forms; and it is therefore a 
matter of the first importance in the study of fossil Sponges to consider somewhat 
in detail the changes by which their primary structures have been so largely 
modified. 
1. Siliceous Sponges ; the nature of the Silica, and the changes in it during fossiliza- 
tion.—The silica forming the skeletal spicules and spicular mesh of recent Sponges, 
is in the amorphous or colloid condition, beautifully clear, like a perfect glass, and 
quite negative in polarized light, between crossed Nicols. It is also soluble in 
heated caustic potash. It is very rare to find the silica of fossil spicules retaining 
the same brilliant glassy appearance and structure as in the recent forms; and, so 
far as I am aware, this perfect condition of preservation is only found in detached 
spicules and fragments of spicular mesh, which have been preserved in a matrix of 
fossil diatomaceous or radiolarian earth of Tertiary age. Spicules in this condi- 
