Fig. 156.—Trionyx, or Turtle, of the Tertiary period.
In the class Fishes we now see the Pleuronectes, or flat-fish, of which Platax altissimus and Rhombus minimus are well-known examples. Among the Crustaceans we see the earliest crabs. At the same time multitudes of new Mollusca make their appearance: Oliva, Triton, Cassis, Harpa, Crepidula, &c.
XXIII.—Ideal Landscape of the Eocene Period.
The hitherto unknown forms of Schizaster are remarkable among Echinoderms; the Zoophytes are also abundant, especially the Foraminifera, which seem to make up by their numbers for their deficiency in size. It was in this period, in the bosom of its seas, and far from shore, that the Nummulites existed, whose calcareous envelopes play such a considerable part as the elements of some of the Tertiary formations. The shelly agglomerates of these Protozoan Rhizopods constitute now very important rocks. The Nummulitic limestone forms, in the chain of the Pyrenees, entire mountains of great height; in Egypt it forms strata of considerable extent, and it is of these rocks that the ancient pyramids were built. What an enormous time must have been necessary to convert the remains of these little shells into beds many hundreds of feet thick! The Miliola were also so abundant in the Eocene seas as to constitute the greater part of calcareous rocks[83] out of which Paris has been built. Agglomerated in this manner, these little shells form the continuous beds of limestone which are quarried for building purposes in the environs of Paris, at Gentilly, Vaugirard, and Châtillon.