FOSSIL SPONGES, ETC. * 29 



aware that the most lowly organized animals and 

 plants are those which have had the v/idest dis- 

 tribution, both in time and space. It is the most 

 highly organized species of animals and plants which 

 best mark the geological ages of formations. So that 

 the fact of finding " Eozoonal structure " in limestones 

 other than the Laurentian, is of itself no evidence 

 against the animal nature of the Eozoon. 



In many places such limestones as the Carboni- 

 ferous do not show visible traces of fossils. I have 

 frequently found, in such cases, that a prepared section 

 of such a rock shows it to be unusually rich in 

 foraminifera. In some parts of the world limestones 

 have been almost wholly composed of the shells of 

 these lowly organized animalcules, such as the Fusu- 

 lina Carboniferous limestones of Russia and North 

 America, the Nummulite limestones, of which the 

 Egyptian Pyramids are built, etc. In England, our 

 white chalk is very largely composed of foraminiferal 

 remains, chiefly Globigerina, represented by species 

 which are still living in the Atlantic and Pacific, where 

 the recent dead shells are accumulating and decom- 

 posing and forming a similar bed of chalky ooze 

 on the ocean-floors. Some species of foraminifera, 

 such as the recent Webbina rugosa, have been found 

 in the fossil state in the Lias rocks, so that they 

 have been in continuous existence ever since. One 

 species, Saccamina Carteri, forms almost entire beds 

 of limestone in the north of England and the south of 



