214 



PALEONTOLOGY. 



ture of the sponge is indeed not always preserved, but the 

 general form the sessile or spread- 

 ing character is retained and in 

 some the vents and pores are exhi- 

 bited. Like the bodies of recent 

 Spongiaria, these silicious fossils 

 contain Infusoria in their interior ; 

 and sections of flints prepared by 

 the lapidary, and mounted on glass 

 slides with Canada balsam, exhibit 

 their microscopic forms in great 

 perfection. The spiculse of fresh- 

 water sponges are found in great 



FIG. 149. Ventricuiite siiicified. profusion in lacustrine tertiary 

 beds, along with the shields of 



fossil infusoria. Many of the moss agates are of spongious 



origin ; and the beautiful sections cut from the pebbles of 



FIG. 150. Siiicified Spongiaria from the Kentish chalk.* 



the Sussex coast, and those of the Isle of Wight, owe their 



* Fig. 150. Flint, with plates and spines of Cidaris embedded whilst the 

 silex was fluid. 



