The life of the Cretaceous displays so great an advance over that of the Jurassic that the change may fairly be called a revolution.

Plants

If the separation between the Mesozoic and Cenozoic eras were made entirely with reference to the plants, it would pass between the Lower and the Upper Cretaceous, just as a similar criterion would remove the Upper Permian to the Mesozoic. The vegetation of the Lower Cretaceous, especially of the lowest, is still much like that of the Jura. Ferns, Horsetails, Cycads, and Conifers continue to make up most of the flora. The Cycadales, in particular, abound in the Lower Cretaceous, many of them belonging to the Bennettiteae. On the other hand, the impending revolution is announced by the appearance of Dicotyledons of archaic and primitive type. In the higher parts of the Potomac the Cycads become less abundant and the Dicotyledons very much more so. Here we find many leaves which belong to genera that cannot be distinguished from those of modern forest trees, such as Sassafras, Populus, Liriodendron, etc. No Dicotyledons have been found in the Kootanie of the Northwest, or in the Wealden of northern Europe, but they occur in the Lower Cretaceous of Portugal, Greenland, and Spitzbergen. In the latter part of the Lower and in all the Upper Cretaceous of North America the flora assumes an almost completely modern character, and nearly all of our common kinds of forest trees are represented: Sassafras, Poplars, Willows, Oaks, Maples, Elms, Beeches, Chestnuts, and very many others.

A new element is the Monocotyledonous group of Palms, which speedily assumes great importance. Each successive plant-bearing horizon of the Cretaceous is characterized by its own special assemblage of plants, but in its general features the Upper Cretaceous flora is essentially modern, and this is true of the world at large, while in the Lower division it was only in North America and a few other scattered areas that the Angiosperms had gained a foothold. Cretaceous animals are sufficiently different from those of the Jura, but the change is not so revolutionary as we have found among the plants.

Fig. 299. - Cretaceous leaves, Dakota stage. 1. Dammarites emarginatus Lesq., 1/2. 2. Betulites Westi Lesq., 3/4. 3. Liriodendron giganteum Lesq., 1/2. (Lesquereux).

Plate XV. - Cretaceous Invertebrate Fossils.

Fig. i, Frondicularia major Bornemann, X 3, Up. Cret , N.J. 2, Haplophragmium concavum Bagg, x 12, U. C. 3, Cardiaster cintus Morton, X 1/2, U. C. 4, Terebratella plicata Say, x 1, U. C. 5, Terebratula karlani Whitf. x 1/2, U. C. 6, Inoceratuus vanuxemi M. and H , x 1/2, Ft. Pierre. 7, Aucella pioche Gabb, X 1, Knoxville. 8, [donearca nebrascensis Owen, X 1/2, Fox Hills. 9, Exogyra costata Say, x 3/8, U. C. 10, Vem'ella conradi Morton, X 5/8, U. C. 11, Ostrea larva Lam., x 5/8, U. C 12. Pyropsis bairdi M. and H , X 1/2, Fox Hills. 13, Anchura americana x 1, Evans and Shumard, Fox Hills 14, Baculites compressus S y, X 1/2, fragment of adult with suture-lines, Ft Pierre. 15, The same, a very young shell, x 5, showing apical coil. 16, Scaphites nodosus Owen, x 3/8, Ft. Pierre 17, Heteroceras stevensoni Whitf., x 2/5, Ft. Pierre, 18, Belemnitella americana Morton, x 1/2, U. C.

Foraminifera play an important part in the construction of Cretaceous rocks, especially of the great masses of chalk (PI. XV, Figs, 1, 2), while the green sands are casts of foraminiferal shells in glauconite. The most abundant genus, as in the recent Atlantic oozes, is Globigerina.

Spongida

In the Cretaceous of Europe Sponges are more numerous and varied than at any other time, but in North America they are far less common.

Ccelenterata

The Corals were very much as they are to-day and require no special description.

The Echinodermata undergo some very marked changes. The Crinoids are much reduced since the Jurassic, and were never again to assume their ancient importance; characteristic Cretaceous genera are the stemless, free-swimming Uintacrinus and Marsupites. The Sea-urchins are incomparably more numerous in Europe than in North America; of the Regular forms may be mentioned Pseudodiadema, Cidaris, and Salenia, and of the Irregular forms, Toxaster, Holaster, Cassidulus, Cordiaster (XV, 3), etc.

Arthropoda

Among the Crustacea we need only note the great increase in the Brachyuran Decapods, or Crabs, in the beds of the Gulf border and of Europe.

Brachiopoda are very much as in the Jurassic; the common genera are Terebratula (XV, 5), Terebratella (XV, 4), and Rhyn-chonella.

Mollusca

This group is very richly developed, and many genera are peculiar to the period. The large, curious oysters belonging to the genera Ostrea (XV, 11), Gryphcea, and especially Exogyra (XV, 9), are common, and the many species of Inocera-mus (XV, 6) are very characteristic, especially of the northern facies. More modern types are Idonearca (XV, 8) and Veniella (XV, 10). Confined to the Cretaceous are the extraordinary shells classed as Rudistes, in which one valve is long and horn-shaped, and the other a mere cover for it. These shells of the genera Hippurites, Radiolites, and Coralliochama are much commoner in Europe than in America and are preeminently southern in distribution. Other peculiar Cretaceous Bivajves are Requienia and Caprotina. Aucella (XV, 7) is of interest both in the Upper Jurassic and the Cretaceous as a typically Boreal group of shells. The Gastropods (XV, 12, 13), are very much as in the Jura, but in the latter part of the period come in many genera which reach their fullest development in Tertiary and recent times, such as Fusus, Murex, Voluta, Cyprcea, and many others. .

The Cephalopods are very peculiar; in addition to numerous Ammonoid genera with closely coiled shells of normal type, such as Hoplites, Schloenbachia, Placenticeras, we find very many shells entirely or partially uncoiled, or rolled up in peculiar ways, which give to the Cretaceous Cephalopod fauna a character all its own. In Crioceras (XIV, 20) the shell is coiled in an open, flat spiral, the whorls of which are not in contact. Ancyloceras has a similar open coil, followed by a long, straight portion, and recurved terminal chamber. Scaphitqs (XV, 16) is like a shortened Ancyloceras. In Ptychoceras the shell consists of two parallel parts, connected by a single sharp bend. Turrilites is coiled into a high spiral, like a Gastropod, and Baculites (XV, 14, 15) has a perfectly straight shell except for a minute coil at one end. Heteroceras (XV, 17) displays the extreme of irregularity of growth. Nautilus is represented by many species, some of them very large. Belem-nites are very abundant, but in the Upper Cretaceous the genus Belemnitella (XV, 18) replaces the true Belemnites.

The Vertebrata

The Vertebrata form the most characteristic element of the Cretaceous fauna. Among the Fishes a revolution has occurred. Sharks of modern type abound, and their teeth are found in countless numbers; but the principal change consists in the immense expansion of the Teleosts, or Bony Fishes, which now take the dominant place, while Ganoids become rare. Most of the Cretaceous Teleosts belong to modern families and even genera, such as the Herrings, Cod, Salmon,Mullets, Catfishes, etc.; but a characteristic Cretaceous type, now extinct, is that of the Saurodonts, fierce, carnivorous fishes of great size and power. The genus Portheus, common in the Kansas chalk, was 12 to 15 feet long, and was provided with great reptile-like teeth.

The Reptiles continued to be the dominant types of the land, the sea, and the air, and it may fairly be questioned whether the Jura or the Cretaceous should be regarded as the culminating period of Reptilian history. Ichthyosaurs and Plesiosaurs are perhaps less abundant than in the Jura, but are of greatly increased size. Elasmosaurus, a Plesiosaur from the Kansas chalk, had a length of 40 to 50 feet, of which 22 feet belonged to the slender neck. Confined to the Cretaceous are the remarkable marine reptiles of the group Mosasauria, which swarmed on the Atlantic and Gulf coasts, and especially in the interior sea. These were gigantic, carnivorous marine lizards, with the limbs converted into swimming paddles (see Fig. 300). Turtles, both fresh-water and marine, abound, and some were very large. Lizards and Snakes are but scantily represented, not displaying the manifold variety of structure which they afterwards acquired. An order of aquatic reptiles, the Choristodera, nearly allied to the Rhyncho-cephalia, appeared in the latter part of the period.

Crocodiles, like those of modern days, were ubiquitous in both fresh and salt waters, and in North America, at least, some of the long-snouted Jurassic type of crocodiles, Teleosaurus, continued into the Upper Cretaceous.

Fig. 300. - Tylosaurus dyspelor Marsh. A Mosasaurian in pursuit of Saurodont Fishes {Portheus). Restoration by C. R. Knight, under the direction of Professor H. F. Osborn. (Copyright by Amer. Mus. Nat. Hist., N.Y).

The Pterosaurs of the Cretaceous are remarkable for their great size, far exceeding that of the Jurassic species. The closely allied genera, Ornithostoma of Europe and Pteranodon of Kansas, had a head of nearly 3 feet in length, with a long, pointed, toothless bill, like that of a bird; the spread of wings exceeded 20 feet.