, SPONGIDA. / 141 



often met with in the rocks, are of comparatively little use to 

 the palaeontologist so far as enabling him to classify the fossil 

 forms is concerned, since it is rarely possible to refer them 

 to the Sponge to which they originally belonged. They are, 

 however, of great use in the determination of the living 

 types. The spicules of the true skeleton, on the other hand, 

 are usually united to one another by a sort of articulation, or 

 become cemented together by a deposit of glassy silica ; so 

 that the skeleton forms a more or less continuous framework, 

 admirably adapted for preservation in a fossil condition. 



Until of late years very little was known, with any pre- 

 cision, as to the structure, affinities, or real nature of a great 

 many fossil Sponges, which are now recognised as belonging 

 to the group at present under consideration. This arose partly 

 from the fact that the value of the microscope in palaeon- 

 tology had not been recognised, and partly from the fact 

 that the structure of the living types has only recently been 

 at all fully understood ; while the state of preservation in 

 which these fossils often occur was such as almost inevitably 

 to lead to misconceptions as to their nature, and to give rise to 

 difficulties which are even yet not fully cleared up. It was 

 known, namely, that Sponges with a siliceous skeleton were of 

 common occurrence in various formations, and especially in 

 the Jurassic and Cretaceous rocks ; but it was generally sup- 

 posed that in these cases the skeleton had been originally 

 composed of lime or of horn, which had in the process of 

 ibssilisation been dissolved away and replaced by flint. These 

 Sponges, in fact, were supposed to have undergone silicification 

 a change to which we know that fossils are very often sub- 

 jected. In the face of the now recognised fact that the 

 minute structure of these fossil forms agrees perfectly with 

 that of living siliceous Sponges, and differs wholly from that 

 of any living types of calcareous or horny Sponges, we can- 

 not doubt that their skeleton was primarily composed of 

 flint. We are thus compelled to believe that in many 

 instances the original siliceous skeleton has been more or 

 less completely dissolved away, the space which it originally 

 occupied in the rock being left permanently vacant, or being 

 simultaneously or subsequently filled up with peroxide of iron, 



