24 -PRINCIPLES OF PALEONTOLOGY. 
This is the case, for example, with the so-called ‘“ Crinoidal 
Limestones ” and “ Encrinital Marbles” with which the geolo- 
gist is so familiar, especially as occurring in great beds amongst 
the older formations of the earth’s crust. ‘These are seen, on 
weathered or broken surfaces, or still better in polished slabs 
(fig. 9), to be composed more or less exclusively of the broken 
Fig. 9 —Slab of Crinoidal marble, from the Carboniferous limestone of Dent, in York- 
shire, of the natural size. The polished surface intersects the columns of the Crinoids at 
different angles, and thus gives rise to varying appearances. (Original.) 
stems and detached plates of sea-lilies (Crinoids). Similarly, 
other limestones are composed almost entirely of the skeletons 
of corals; and such old coralline limestones can readily be 
paralleled by formations which we can find in actual course of 
production at the present day. We only need to transport 
ourselves to the islands of the Pacific, to the West Indies, or 
to the Indian Ocean, to find great masses of lime formed simi- 
larly by living corals, and well known to every one under the 
name of “coral-reefs.” Such reefs are often of vast extent, 
both superficially and in vertical thickness, and they fully equal 
in this respect any of the coralline limestones of bygone ages. 
Again, we find other limestones—such as the celebrated 
‘“Nummulitic Limestone” (fig. 10), which sometimes attains a 
thickness of some thousands of feet—which are almost entirely 
made up of the shells of Foraminifera. In the case of the 
‘““Nummulitic Limestone,” just mentioned, these shells are of 
large size, varying from the size of a split pea up to that ofa 
