THE CRETACEOUS PERIOD. 



26 5 



ter dyed with peroxide of iron." (Sir Wyville Thomson.) 

 Many of the Chalk sponges, originally calcareous, have been 

 converted into flint subsequently ; but the Ventriculites are 



Fig. I'&.S 

 Upper Greensand, E 



Fig. i&).re*tricuStts simple* 

 White Chalk, Britain. 



really composed of this substance, and are therefore genuine 

 " Siliceous Sponges," like the existing Venus's Flower-Basket 

 (Euplectella). Like the latter, the skeleton was doubtless ori- 

 ginally composed, in the young state, of disconnected six- 

 rayed spicules, which ultimately become fixed together to 

 constitute a continuous frame-work. The sea-water, as in the 

 recent forms, must have been admitted to the interior of the 

 Sponge by numerous apertures on its exterior, subsequently 

 escaping by a single large opening at its summit. 



Amongst the Cxlenterates, the " Hydroid Zoophytes " are 

 represented by a species of the encrusting genus Hydractinia, 

 the horny polypary of which is so commonly found at the 

 present day adhering to the exterior of shells. The occurrence 

 of this genus is of interest, because it is the first known instance 

 in the entire geological series of the occurrence of an unques- 

 tionable Hydroid of a modern type, though many of the exist- 

 ing forms of these animals possess structures which are per- 



