inches in length, resembling somewhat the farm-fence lizards of to-day. In the Permian era, closing the Pelæozoic realm, other cotylosaurs appeared in numbers, first of moderate size, then gradually increasing in bulk. Of these were the larger Theromera, such as the lizardlike Dimetrodons, some with forty-inch spines on their backs, and the shell-backed, lizardlike Otocœlidæ, the ancestors of the marine turtles (Testudinata), ranging from three to ten feet in length. The Mesozoic realm, following, was the most extraordinary period of earth for its forms of animal life. In its first section, the Triassic, the saurians that appeared assumed wide orders, functions, and greater proportions. These lizardlike saurians were termed Palæoctonus, Ætosaurus, Dystrophacus, Thecodontosaurus, and Palæohatteria.
The Triassic, Jurassic, and Cretaceous formations, following, formed the most remarkable period of types of animal life. An examination shows that the marine Jurassic beds of Colorado were overlaid by a former fresh-water area, or lake, in which abounded iguanodons, megalosaurs, and cetiosaurs of diverse species, which swarmed in vast numbers and grew to gigantic proportions. The forests and jungles abounded in saurians which walked upon four legs and were so armed as to preserve peace while they fed on vegetation. The Cretaceous agathaumas resembled somewhat in form the rhinoceros, and the Jurassic stegosaur partook of the bulk of the elephant.
Under side of Skull of Tylosaurus proriger (Cope).
Agathaumas had a powerful skull, with two long horns over the eyes pointing upward and one over the nose pointing forward. The stegosaurs had a wonderful dermal armor of plates and spines arising along the spinal column. Such were the more specialized types of the forest. The plains and fields swarmed with saurians which ran or leaped on two hind legs and tail, tripod fashion, and whose fore limbs were used for seizing prey or supports. Those of the kangaroo type are represented by the Laramie Cretaceous Lælaps incrassatus
