gS AN INTRODUCTION TO THE STUDY OF FOSSILS 



the silica is in a colloidal condition ; during fossilization this 

 becomes crystalline, or is dissolved and carried off by percolat- 

 ing waters or is replaced by calcium carbonate or some other 

 substance. Likewise the calcareous spicules are often replaced 

 by silica. Thus originally siliceous spicules may be converted 

 into calcite and calcareous spicules may become silicified. 

 Hence the distinction between siliceous and calcareous sponges 

 in the fossil state depends upon form and not upon chemical 

 composition. 



When the siliceous spicules are so loosely bound together 

 as to fall apart upon the decomposition of the soft parts of the 

 animal, they will collect in the depositing sediment and where 

 abundant form porous siliceous beds. Usually the spicules 

 have undergone partial solution in sea- water ; this dissolved 

 silica being redeposited as siliceous cement binds the remaining 

 spicules together. Since the spicules are so very small their 

 presence is detected only by an examination of thin sections of 

 the chert under a high power microscope. The silica from such 

 dissolved spicules and from other siliceous organisms is fre- 

 quently redeposited as nodules of flint. The flint nodules 

 in the Cretaceous chalk beds of England and France are notable 

 examples. 



Sponges are known from the pre-Cambrian to the present. 



Derivation of name. — Porifera > Latin porus, a pore, + 

 ferre, to bear ; referring to the many minute incurrent canals 

 penetrating the wall of the sponge. 



The Porifera may be divided into the following sub-classes : 



Page 



A. Calcarea 102 



B. Non-Calcarea 102 



The old sub-class Silicispongiae include the Hexactinellida, 

 TetractineUida and Monactinellida ; while the apparently more 

 genetic sub-class Desmospongiae includes the TetractineUida, 

 Monactinellida, Ceratospongida and Myxospongida. Accord- 

 ing to this latter classification all sponges would be divided into 

 Calcarea, Hexactinellida and Desmospongiae. 



