26 PRINCIPLES OF PALEONTOLOGY. 



shells are of a large size, varying from the size of a split pea up 

 to that of a florin. There are, however, as we shall see, many 

 other limestones, which are likewise largely made up of 



Fig. 10. Piece of Nummulitic Limestone from the Great Pyramid. 

 Of the natural size. (Original.) 



Foraminifera, but in which the shells are very much more minute, 

 and would hardly be seen at all without the microscope. 



We may, in fact, consider that the great agents in the pro- 

 duction of limestones in past ages have been animals belonging 

 to the Crinoids, the Corals, and the Foraminifera. At the pres- 

 ent day, the Crinoids have been nearly extinguished, and the 

 few known survivors seem to have retired to great depths in the 

 ocean ; but the two latter still actively carry on the work of 

 lime-making, the former being very largely helped in their 

 operations by certain lime-producing marine plants (Nullipores 

 and Corallines}. We have to remember, however, that though 

 the limestones, both ancient and modern, that we have just 

 spoken of, are truly organic, they are not necessarily formed out 

 of the remains of animals which actually lived on the precise 

 spot where we now find the limestone itself. We may find a 

 crinoidal limestone, which we can show to have been actually 

 formed by the successive growth of generations of sea-lilies in 

 place; but we shall find many others in which the rock is made 

 up of innumerable fragments of the skeletons of these creatures, 

 which have been clearly worn and rubbed by the sea-waves, and 

 which have been mechanically transported to their present site. 



