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. In the genera 

 Elasmostoma 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. 



Chemical Constitution and Mode op Preservation op Fossil Sponges. 



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- 



