What a strange population was that which occupied the earth at this stage of its history, when the waters were filled with creatures so extraordinary as those whose history we have traced! Plesiosauri and Ichthyosauri filled the seas, upon the surface of which floated innumerable Ammonites in light skiffs, some of them as large as a good-sized cart-wheel, while gigantic Turtles and Crocodiles crawled on the banks of the rivers and lakes. Only one genus of Mammals had yet appeared, but no birds; nothing broke the silence of the air, if we except the breathing of the terrestrial reptiles and the flight of winged insects.

The earth cooled progressively up to the Jura.s.sic period, the rains lost their continuity and abundance, and the pressure of the atmosphere sensibly diminished. All these circ.u.mstances favoured the appearance and the multiplication of innumerable species of animals, whose singular forms then showed themselves on the earth. We can scarcely imagine the prodigious quant.i.ty of Molluscs and Zoophytes whose remains lie buried in the Jura.s.sic rocks, forming entire strata of immense thickness and extent.

[Ill.u.s.tration: Fig. 107.--Draco volans.]

The same circ.u.mstances concurred to favour the production of plants. If the sh.o.r.es and seas of the period received such a terrible aspect from the formidable animals we have described, the vegetation which covered the land had also its peculiar character and appearance. Nothing that we know of in the existing scenery of the globe surpa.s.ses the rich vegetation which decorated the continents of the Jura.s.sic period. A temperature still of great elevation, a humid atmosphere, and, we have no reason to doubt, a brilliant sun, promoted the growth of a luxuriant vegetation, such as some of the tropical islands, with their burning temperature and maritime climate, can only give us an idea of, while it recalls some of the Jura.s.sic types of vegetation. The elegant Voltzias of the Trias had disappeared, but the Horse-tails (_Equiseta_) remained, whose slender and delicate stems rose erect in the air with their graceful panicles; the gigantic rushes also remained; and though the tree-ferns had lost their enormous dimensions of the Carboniferous age, they still preserved their fine and delicately-cut leaves.

Alongside these vegetable families, which pa.s.sed upwards from the preceding age, an entire family--the Cycads (Fig. 72, p. 168)--appear for the first time. They soon became numerous in genera, such as Zamites, Pterophyllum (Williamsonia), and Nilssonia. Among the species which characterise this age, we may cite the following, arranging them in families:--

FERNS. CYCADS. CONIFERS.

Odontopteris cycadea. Zamites distans. Taxodites.

Taumopteris Munsteri. Zamites heterophyllus. Pinites.

Camptopteris crenata. Zamites gracilis.

Pterophyllum dubium.

Nilssonia contigua.

Nilssonia elegantissima.

Nilssonia Sternbergii.

The _Zamites_ seem to be forerunners of the Palms, which make their appearance in the following epoch; they were trees of elegant appearance, closely resembling the existing Zamias, which are trees of tropical America, and especially of the West India Islands; they were so numerous in species and in individuals that they seem to have formed, of themselves alone, one half of the forests during the period which engages our attention. The number of their fossilised species exceeds that of the living species. The trunk of the Zamites, simple and covered with scars left by the old leaves, supports a thick crown of leaves more than six feet in length, disposed in fan-like shape, arising from a common centre.

The _Pterophyllum_ (Williamsonia), formed great trees, of considerable elevation, and covered with large pinnated leaves from top to bottom.

Their leaves, thin and membranous, were furnished with leaflets truncated at the summit and traversed by fine nervures, not convergent, but ab.u.t.ting on the terminal truncated edge.

The _Nilssonia_, finally, were Cycadeaceae resembling the Pterophyllum, but with thick and coriaceous leaves, and short leaflets contiguous to, and in part attached to the base; they were obtuse or nearly truncated at the summit, and would present nervures arched or confluent towards that summit.

The essential characters of the vegetation during the Lia.s.sic sub-period were:--1. The great predominance of the Cycadeaceae, thus continuing the development which commenced in the previous period, expanding into numerous genera belonging both to this family and that of the _Zamites_ and _Nilssonia_; 2. The existence among the Ferns of many genera with reticulated veins or nervures, and under forms of little variation, which scarcely show themselves in the more ancient formations.

[Ill.u.s.tration: Fig. 108.--Millepora alcicornis.

(Recent Coral.)]

[Ill.u.s.tration: XVI.--Ideal Landscape of the Lia.s.sic Period.]

On the opposite page (PLATE XVI.) is an ideal landscape of the Lia.s.sic period; the trees and shrubs characteristic of the age are the elegant Pterophyllum, which appears in the extreme left of the picture, and the Zamites, which are recognisable by their thick and low trunk and fan-like tuft of foliage. The large horsetail, or Equisetum of this epoch, mingles with the great Tree-ferns and the Cypress, a Conifer allied to those of our own age. Among animals, we see the Pterodactyle specially represented. One of these reptiles is seen in a state of repose, resting on its hind feet. The other is represented, not flying, after the manner of a bird, but throwing itself from a rock in order to seize upon a winged insect, the dragon-fly (_Libellula_), the remains of which have been discovered, a.s.sociated with the bones of the Pterodactyle, in the lithographic limestone of Pappenheim and Solenhofen.

OOLITIC SUB-PERIOD.

This period is so named because many of the limestones entering into the composition of the formations it comprises, consist almost entirely of an aggregation of rounded concretionary grains resembling, in outward appearance, the roe or eggs of fishes, and each of which contains a nucleus of sand, around which concentric layers of calcareous matter have acc.u.mulated; whence the name, from ???, _egg_, and ?????, _stone_.

The Oolite series is usually subdivided into three sections, the _Lower_, _Middle_, and _Upper Oolite_. These rocks form in England a band some thirty miles broad, ranging across the country from Yorkshire, in the north-east, to Dorset, in the south-west, but with a great diversity of mineral character, which has led to a further subdivision of the series, founded on the existence of particular strata in the central and south-western counties:--

UPPER. MIDDLE. LOWER.

1. Purbeck Beds. 1. Coral Rag. 1. Cornbrash.

2. Portland Stone 2. Oxford Clay. 2. Great Oolite & Forest and Sand. Marble.

3. Kimeridge Clay. 3. Stonesfield Slate.

4. Fuller's Earth.

5. Inferior Oolite.

The alternations of clay and ma.s.ses of limestone in the Lia.s.sic and Oolite formations impart some marked features to the outline of the scenery both of France and England: forming broad valleys, separated from each other by ranges of limestone hills of more or less elevation.

In France, the Jura mountains are composed of the latter; in England, the slopes of this formation are more gentle--the valleys are intersected by brooks, and clothed with a rich vegetation; it forms what is called a tame landscape, as compared with the wilder grandeur of the Primary rocks--it pleases more than it surprises. It yields materials also, more useful than some of the older formations, numerous quarries being met with which furnish excellent building-materials, especially around Bath, where the stone, when first quarried, is soft and easily worked, but becomes harder on exposure to the air.

The annexed section (Fig. 109) will give some idea of the configuration which the stratification a.s.sumes, such as may be observed in proceeding from the north-west to the south-east, from Caermarthenshire to the banks of the Ouse.

LOWER OOLITE FAUNA.

The most salient and characteristic feature of this age is, undoubtedly, the appearance of animals belonging to the cla.s.s of Mammals. But the organisation, quite special, of the first of the Mammalia will certainly be a matter of astonishment to the reader, and must satisfy him that Nature proceeded in the creation of animals by successive steps, by transitions which, in an almost imperceptible manner, connect the beings of one age with others more complicated in their organisation. The first Mammals which appeared upon the earth, for example, did not enjoy all the organic attributes belonging to the more recent creations of the cla.s.s. In the latter the young are brought forth living, and not from eggs, like Birds, Reptiles, and Fishes. But the former belonged to that order of animals quite special, and never numerous, the young of which are transferred in a half-developed state, from the body of the mother to an external pouch in which they remain until they become perfected; in short, to marsupial animals. The mother nurses her young during a certain time in a sort of pouch external to the body, in the neighbourhood of the abdomen, and provided with teats to which the young adhere. After a more or less prolonged sojourn in this pouch, the young animal, when sufficiently matured and strong enough to battle with the world, emerges from its warm retreat, and enters fully into life and light; the process being a sort of middle course between oviparous generation, in which the animals are hatched from eggs after exclusion from the mother's body, like Birds; and viviparous, in which the animals are brought forth alive, as in the ordinary Mammals.

[Ill.u.s.tration: Fig. 109.--General view of the succession of British strata, with the elevations they reach above the level of the sea.

_G_, Granitic rocks; _a_, Gneiss; _b_, Mica-schist; _c_, Skiddaw or c.u.mbrian Slates; _d_, Snowdon rocks; _e_, Plynlymmon rocks; _f_, Silurian rocks; _g_, Old Red Sandstone; _h_, Carboniferous Limestone; _i_, Millstone Grit; _k_, Coal-measures; _l_, Magnesian Limestone; _m_, New Red Sandstone; _n_, Lias; _o_, Lower, Middle, and Upper Oolites; _p_, Greensand; _q_, Chalk; _r_, Tertiary strata.]

In standard works on natural history the animals under consideration are cla.s.sed as _mammiferous Didelphae_. They are brought forth in an imperfect state, and during their transitional condition are suckled in a pouch supported by bones called _marsupial_, which are attached by their extremities to the pelvis, and serve to support the marsupium, whence the animals provided with these provisions for bringing up their progeny are called _Marsupial Mammals_. The Opossum, Kangaroo, and Ornithorhynchus are existing representatives of this group.

[Ill.u.s.tration: Fig. 110.--Jaw of Thylacotherium Prevostii.]

[Ill.u.s.tration: Fig. 111.--Jaw of Phascolotherium.]

The name of _Thylacotherium_, or _Amphitherium_, or _Phascolotherium_, is given to the first of these marsupial Mammals which made their appearance, whose remains have been discovered in the Lower Oolite, and in one of its higher stages, namely, that called the _Great Oolite_.

Fig. 110 represents the jaw of the first of these animals, and Fig. 111 the other--both of the natural size. These jaw-bones represent all that has been found belonging to these early marsupial animals; and Baron Cuvier and Professor Owen have both decided as to their origin. The first was found in the Stonesfield quarries. The Phascolotherium, also a Stonesfield fossil, was the ornament of Mr. Broderip's collection. The animals which lived on the land during the Lower Oolitic period would be nearly the same with those of the Lia.s.sic. The insects were, perhaps, more numerous.

The marine fauna included Reptiles, Fishes, Molluscs, and Zoophytes.

Among the first were the Pterodactyle, and a great Saurian, the Teleosaurus, belonging to a family which made its appearance in this age, and which reappears in the following epoch. Among the Fishes, the Ganoids and Ophiopsis predominate. Among the Ammonites, _Ammonites Humphriesia.n.u.s_, _A. Herveyii_ (Fig. 112), _A. Brongniarti_, _Nautilus lineatus_, and many other representatives of the cephalopodous Mollusca.

Among the Brachiopods are _Terebratula digona_ (Fig. 113) and _T.

spinosa_. Among the Gasteropoda the _Pleurotomaria conoidea_ is remarkable from its elegant shape and markings, and very unlike any of the living _Pleurotoma_ as represented by _P. Babylonia_ (Fig. 114).

_Ostrea Marshii_ and _Lima proboscidea_, which belong to the Acephala, are fossil Mollusca of this epoch, to which also belong _Entalophora cellarioides_, _Eschara Ranviliana_, _Bidiastopora cervicornis_; elegant and characteristic molluscous Polyzoa. We give a representation of two living species, as exhibiting the form of these curious beings. (Figs.

115 and 116.)

[Ill.u.s.tration: Fig. 112.--Ammonites Herveyii.]

[Ill.u.s.tration: Fig. 113.--Terebratula digona.]

[Ill.u.s.tration: Fig. 114.--Pleurotoma Babylonia. (Recent.)]

The Echinoderms and Polyps appear in great numbers in the deposits of the Lower Oolite: _Apiocrinus elegans_, _Hyboclypus gibberulus_, _Dysaster Endesii_ represent the first; _Montlivaltia caryophyllata_, _Anabacia orbulites_, _Cryptocnia bacciformis_, and _Eunomia radiata_ represent the second.

[Ill.u.s.tration: Fig. 115.--Adeona folifera.

(Recent Polyzoa.)]

[Ill.u.s.tration: Fig. 116.--Cellaria loriculata.

(Recent Polyzoa.)]

This last and most remarkable species of Zoophyte presents itself in great ma.s.ses many yards in circ.u.mference, and necessitates a long period of time for its production. This a.s.semblage of little creatures living under the waters but only at a small depth beneath the surface, as Mr.