reader a little illustration of the limestone-making Foraminifera of the Cretaceous and Eocene seas. In the middle above is a nummulite of the natural size. Below is another, sliced to show its internal chambers. At one side is a magnified section of the common building stone of Paris, the milioline limestone of the Eocene, so called from its immense abundance of microscopic shells of the genus Miliolina. At the other side is a magnified section of one of the harder varieties of chalk, ground so thin as to become transparent,[AH] and mounted in Canada balsam. It shows many microscopic chambered shells of Foraminifera. These may serve as illustrations of the functions of these humble inhabitants of the sea as accumulators of calcareous matter. It is further interesting to remark that some of the beds of nummulitic limestone are so completely filled with these shells, that we might from detached specimens suppose that they belonged to sea-bottoms whereon no other form of life was present. Yet some beds of this age are remarkably rich in other fossils. Lyell states that as many as six hundred species of shells have been found in the principal limestone of the Paris basin alone; and the lower Eocene beds afford remains of fishes, of reptiles, of birds, and of mammals. Among the latter are the bones of gigantic whales, of which one of the most remarkable is the Zeuglodon of Alabama, a creature sometimes seventy feet in length, and which replaces in the Tertiary the great Elasmosaurs and Ichthyosaurs of the Mesozoic, marking the advent, even in the sea, of the age of Mammals as distinguished from the age of Reptiles. [AH] As for instance that of the Giant's Causeway, Antrim. [Illustration: FORAMINIFERAL ROCK-BUILDERS. A. Nummulites lævigata--Eocene. B. The same, showing chambered interior. C. Milioline limestone, magnified--Eocene, Paris. D. Hard Chalk, section magnified--Cretaceous.] This fact leads us naturally to consider in the second place the mammalia, and other land animals of the Tertiary. At the beginning of the period we meet with that higher group of mammals, not pouched, which now prevails. Among the oldest of these Tertiary beasts are _Coryphodon_, an animal related to the Modern Tapirs, and _Arctocyon_, a creature related to the bears and racoons. These animals represent respectively the Pachyderms, or thick-skinned mammals, and the ordinary Carnivora. Contemporary with or shortly succeeding these, were species representing the Rodents, or gnawing animals, and many other creatures of the group Pachydermata, allied to the Modern Tapirs and Hogs, as well as several additional carnivorous quadrupeds. Thus at the very beginning of the Tertiary period we enter on the age of mammals, It may be well, however, to take these animals somewhat in chronological order.