THE CRETACEOUS PERIOD. 



273 



ridged or grooved, or otherwise ornamented on the surface, 

 frequently expanded above into a cup-like lip, and continued 

 below into a bundle of fibrous roots. The minute structure of 

 these bodies shows an extremely delicate tracery of fine tubes, 

 sometimes empty, sometimes filled with loose calcareous mat- 

 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 

 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 orig- 

 inally 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 admilted to the interior of the 

 Sponge by numerous apertures on its exterior, subsequently 

 escaping by a single large opening at its summit. 



Fig. ISS.Sipfionia flcu. 

 Upper Greensand, Europe. 



Fig. ISQ.VentricuUtes simplex. 

 White Chalk, Britain. 



Amongst the Ccelentcrates, the " Hydroid Zoophytes " are 

 represented by a species of the encrusting genus Hydractinia, 

 the horny polypary of which is so commonly found at the 

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