472 



MESOZOIC TIME. 



Sponges, also, were of great importance in the history of the 

 Cretaceous rocks. They occur cup or saucer-shaped, tubular, branched, 

 and of other forms. One is figured in Fig. 860. Their siliceous 

 spicula (Fig. 861 a-g) are common in the flint, and have contributed, 

 as well as Diatoms, toward the silica of which it was made. The 

 recent discovery over the ocean's bottom of sponges whose fibres are 

 wholly siliceous, shows that these species may have contributed much 

 to flint-making. The Ventriculites of the chalk are supposed to have 

 been siliceous Sponges. 



Among Radiates, the Corals and Echinoids were mostly of modern 

 types. 



The same genera of Mollusks abounded that are enumerated on p. 

 460. The genera of Gasteropods were to a greater extent modern 

 genera than in the preceding period ; and the proportion of siphonated 



Figs. 862-866. 



Covchifers, Rudistes Family.— Fig. 862. Hippurites Toucasianus; 862 a, H. dilatatus ; 863, 

 Radiolites Bournoni ; 864, Spherulites Hoeninghausi. Gasteropods. — 865, Nerinsea bisulcata ; 

 866, Avellana Cassis. 



species (those having a beak) was nearly as great as in existing seas. 

 The Rudistes (Figs. 862-866) were very common in southern Europe 

 and Asia Minor ; and about eighty species have been described. Only 

 a single species — Radiolites Mortoni Woodw. — has been found in 



