The most salient and characteristic feature of this age is, undoubtedly, the appearance of animals belonging to the class 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 class. 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. [Illustration: 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 Cumbrian 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 classed as _mammiferous Didelphæ_. 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. [Illustration: Fig. 110.--Jaw of Thylacotherium Prevostii.] [Illustration: Fig. 111.--Jaw of Phascolotherium.]