SPONGIDA. 143 



being the beautiful Venus's Flower-basket (Euplectella). Now 

 we not only know of a long series of living Sponges with a 

 flinty skeleton built upon the plan above defined, but we 



Fig. 38. A, Side-view of a specimen of Cceloptychium Seebachi, of the natural size, from 

 the Cretaceous formation ; B, Portion of the hexradiate skeleton of the same, enlarged 65 

 times. (After Zittel.) 



know that the group enjoyed a great development in past 

 time, and that it can be traced back to the Silurian period. 

 All the recent Hexactinellidce are inhabitants of the more 

 profound depths of the ocean, and are in no case strictly 

 shallow-water forms. It is probable, therefore, that the fossil 



