With the Stonisfield slates,--a deposit which lies above what is known as the Inferior Oolite,--the remains of mammaliferous animals first appear. As, however, no other mammalian remains occur until after the close of the great Secondary Division, and as certain marked peculiarities attach to these Oolitic ones, it may be well to inquire whether their place, so far in advance of their fellows, may not be indicative of a radical difference of character,--a difference considerable enough to suggest to the zoologist an improvement in his scheme of classification. It has been shown by Professor Owen,--our highest authority in comparative anatomy,--that while one Stonisfield genus unequivocally belonged to the marsupial order, another of its genera bears also certain of the marsupial traits; and that the group which they composed,--a very small one, and consisting exclusively of minute insect-eating animals,--exhibits in its general aspect the characteristics of this pouched family. Even the genus of the group that least resembles them was pronounced by Cuvier to have its nearest affinities with the opossums. And let us mark how very much may be implied in this circumstance. In the "_Animal Kingdom_" of the great naturalist just named, the marsupiata, or pouched animals, are made to occupy the fourth place among the nine orders of the Mammalia; but should they not rather occupy a place intermediate between the placental mammals and the birds? and does not nature indicate their true position by the position which she assigns to them in the geologic scale? The birds are oviparous; and between the extrusion of the egg and the development of the perfect young bird they have to hatch it into life during a long period of incubation. The marsupiata are not oviparous, for their _eggs_ want the enveloping shell or skin; but they, too, are extruded in an exceedingly rudimentary and foetal state, and have to undergo in the pouch a greatly longer period of _incubation_ than that demanded by nature for any bird whatever. The young kangaroo is extruded, after it has remained for little more than a month in the womb, as a foetus scarcely an inch in length by somewhat less than half an inch in breadth: it is blind, exhibiting merely dark eye spots; its limbs are so rudimentary, that even the hinder legs, so largely developed in the genus when mature, exist as mere stumps; it is unable even to suck, but, holding permanently on by a minute dug, has the sustaining fluid occasionally pressed into its mouth by the mother. And, undergoing a peculiar but not the less real process of incubation, the creature that had to remain for little more than a month in the womb,--strictly thirty-nine days,--has to remain in the mother's pouch, ere it is fully developed and able to provide for itself, for a period of eight months. It is found to increase in weight during this hatching process, from somewhat less than an ounce to somewhat more than eight pounds.