THE CRETACEOUS PERIOD. 265 
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. 188.—Szphonia ficus. Fig. 189.—Ventriculites simplex. 
Upper Greensand, Europe. White Chalk, Britain. 
really composed of this substance, ard are therefore genuine 
“‘Siliceous Sponges,” like the existing Venus’s Flower-Basket 
(Zuplectella). 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 Calenterates, the “Hydroid Zoophytes” are 
represented by a species of the encrusting genus Wydractinia, 
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- 
