CRETACEOUS ERA. 



instance, upon a piece of decaying organic matter, as lias been 

 the case with the nodules of ironstone in the earlier rocks, and 

 the spherules of the oolite. 



FIG. 62. 



Siliceous- shelled Infusoria. 

 A, Coscinodiscus ; B, Actinocyclus; C, Triceratium; D. Biddulphia. 



What is more remarkable, M. Ehrenberg has ascertained 

 that at least fifty-seven species of the microscopic animals of 

 the chalk, being infusoria and calcareous-shelled polythalamia, 

 are still found living in various parts of the earth. These 

 species are the most abundant in the rock. Singly they are 

 the most unimportant of all animals ; but in the mass, forming 

 as they do such enormous strata over a large part of the earth's 

 surface, they have an importance greatly exceeding that of the 

 largest and noblest of the beasts of the field. Moreover, these 

 species have a peculiar interest, as the only specific types of 

 that early age which have survived to the present day. While 



