These creatures abound in all parts of the ocean, from the surface to the greatest depths. The Foraminifera have also existed from the earliest geological times, and in all the long ages of the earth's history seem to have retained the same structures and even ornamentation; so that species from very old geological formations are often scarcely distinguishable from those now living, and must have played precisely the same parts in the system of nature. One of these functions is that of accumulating great thicknesses of calcareous matter in the sea-bottom. The manner in which such accumulation takes place we learn from what is now going on in the ocean, more especially from the result of the recent deep-sea dredging expeditions. The Foraminifera are vastly numerous, both near the surface and at the bottom of the sea, and multiply rapidly; and as successive generations die, their shells accumulate on the ocean bed, or are swept by currents into banks, and thus in process of time constitute thick beds of white chalky material, which may eventually be hardened into limestone. This process is now depositing a great thickness of white ooze in the bottom of the ocean; and in times past it has produced such vast thicknesses of calcareous matter as the chalk and the nummulitic limestone of Europe and the orbitoidal limestone of America. The chalk, which alone attains a maximum thickness of 1,000 feet, and, according to Lyell, can be traced across Europe for 1,100 geographical miles, may be said to be entirely composed of shells of Foraminifera imbedded in a paste of still more minute calcareous bodies, the Coccoliths, which are probably products of marine vegetable life, if not of some animal organism still simpler than the Foraminifera. There are, however, some sessile examples of these animals which attain to larger dimensions than the free and locomotive forms. As an example of these we may take the _Polytrema_, which forms little hard red lumps on West Indian corals. Such a creature, beginning life as a little round spot of protoplasm, almost invisible, and protected with a little dome of carbonate of lime for the extension of its pseudopods as it grows in size, adds chamber to chamber in successive tiers till it assumes an appreciable size, all the chambers communicating with each other, while the outer ones are perforated with pores for extension of the pseudopods. In one form (_Carpenteria_) the same end is secured by leaving an open space in the middle of the conical mass like the crater of a small volcano. It is with these larger and sessile forms that we must compare Eozoon, though some of its minute structures rather resemble those of some smaller types.