day in the Mesozoic era, for none of them survived beyond this time.

In the Mesozoic we meet with the first remains of birds. The oldest known representative of this class is the Archæopteryx, of which two specimens only have been found in the Jurassic. These specimens are remarkable in many ways. Unlike modern birds, the Archeopteryx had well developed, conical teeth in both jaws; the tail was much elongated, and composed of a large

number of free vertebræ, into each of which was inserted a pair of large feathers, thus presenting a marked contrast to all existing birds, in which the tail is much shortened and has several of the vertebræ fused into one bone. The wing of the Archæopteryx shows an approach toward the primitive pentadactyl type of limb in having three independent digits, each ending in a claw. Comparative anatomists have long recognized the structural resemblances between birds and reptiles. The discovery of Archæopteryx provides a most interesting link connecting these two classes. There are other toothed birds in the Cretaceous period, which immediately follows the Jurassic, but, aside from

the presence of teeth, they show a fairly close resemblance to modern types (Fig. 210).

The Mesozoic era also saw the first introduction of the mammals. The earliest undoubted remains of this group are found in the latter part of the Triassic, although reptiles with marked mammalian affinities occur in the first part of this period. Throughout the long stretch of the Mesozoic era, the mammals remain small and inconspicuous, most of them not exceeding the size of mice and rats. So long as the reptiles dominated the land, the mammals made relatively little progress; but when the last of the great dinosaurs made their dramatic exit from the

FIG. 208-A restoration of Brontosaurus excelsus from the Jurassic of Wyoming. (After Marsh.)

stage of life toward the close of the Cretaceous, the mammals apparently took advantage of their opportunity, and we find them, in the following periods of the Tertiary era, assuming a much larger size, and developing into a large number of diversified forms. The Tertiary is therefore known as "the age of mammals."

The earliest Tertiary mammals contained a considerable number of relatively primitive forms, and some of the orders then represented have since become entirely extinct. Grabau designates them as

archaic animals with extremely small brains, simple triangular teeth, five-toed feet, and flat-footed (plantigrade) mode of progression. They were defective in mental power, ill-adapted in tooth structure for the effective procurement of food, and in general not well fitted for rapid motion because of their flat-footedness. In other respects, they had become very diverse, simulating the structural characters which in the higher types characterize the different groups.

During the succeeding periods of the Tertiary the mammals made a rapid approach toward modern types. In Miocene times, for instance, there are the remains of deer, camels, rhinoceroses, tapirs, elephants, horses, saber-tooth tigers, and wolves, al

FIG. 210-Toothed birds from the Cretaceous, Ichthyornis victor to the left; Hesperornis regalis to the right. (After Marsh.)

though most of these forms belong to different genera from those of the present day. During the Tertiary, the flora became much more modernized. As Agassiz has remarked:

The grains, the Rosacea with their varied fruits, the tropical fruit trees, Oranges, Bananas, etc., the shade and cluster trees so important to the comfort and shelter of man, are added to the vegetable world

during these epochs. The fossil vegetation of the Tertiaries is indeed most interesting in showing the gradual maturing and completion of those conditions most intimately associated with human life. The earth had already its seasons, its spring and summer, its autumn and winter, its seed time and harvest, though neither sower nor reaper was there; the forests then as now dropped their thick carpet of leaves on the ground in autumn, and in many localities they remained where they originally fell with a layer of soil between the successive layers of leaves, a leafy chronology by which we read the passage of the years which divided these deposits from each other. Where the leaves have fallen singly on a clayey soil favorable for receiving such impressions, they have daguerreotyped themselves with the most wonderful accuracy, and the Oaks, Poplars, Willows, Maples, Walnuts, Gum and Cinnamon trees, etc., of the Tertiaries are as well known to us as those of our own time.

An important feature of Tertiary plant life was the development of grassy plains over extensive semi-arid

appeared at that time in several continents. Th ported large numbers of grazing animals, the rema

THE EVOLUTION OF THE HORSE.

Formations in Western United States and Characteristic Type of Horse in Each

Fore Foot

Hind Foot

Teeth

FIG. 211-Stages in the evolution of the horse. (From a Guide Leaflet of the

American Museum of Natural History.)

Three Toes

Side toes
touching the ground;
splint of 5 digit

Three Toes

Side toes

touching the ground

Four Toes

Short

Crowned

without

Cement

Hypothetical Ancestors with Five Toes on Each Foot

and Teeth like those of Monkeys etc.