THYLACOTHERIUM PREVOSTI.
(Stonisfield Slate.)
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 fœtal 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. Now, this surely is a process quite as nearly akin to the incubation of egg-bearing birds as to the ordinary nursing process of the placental mammals; and on the occult but apparently real principle, that the true arrangement of the animal kingdom is that which we find exemplified by the successive introduction of its various classes and orders in the course of geologic history, should we not anticipate a point of time for the introduction of the marsupiata, intermediate between the widely-distant points at which the egg-bearing birds and the true placental mammals appeared? Ranged at once chronologically, and by their mode of reproduction, the various classes of the vertebrata would run, did we accept the suggested reading, as follows:—First appear cold-blooded vertebrates (fishes), that propagate by eggs or spawn,—chiefly by the latter. Next appear cold-blooded vertebrates (reptiles), that propagate by eggs or spawn,—chiefly by the former. Then appear warm-blooded vertebrates (birds), that propagate by eggs exclusively. Then warm-blooded vertebrates come upon the stage, that produce eggs without shells, which have to be subjected for months to a species of extra-placental incubation. And last of all the true placental mammals appear. And thus, tried by the test of perfect reproduction, the great vertebral division receives its full development in creation.
ANOPLTHERIUM COMMUNE.
(Eocene.)
The placental mammals make their appearance, as I have said, in the earliest ages of the great Tertiary division, and exhibit in the group an aspect very unlike that which they at present bear. The Eocene ages were peculiarly the ages of the Palæotheres,—strange animals of that pachydermatous or thick-skinned order to which the elephants, the tapirs, the hogs, and the horses belong. It had been remarked by naturalists, that there are fewer families of this order in living nature than of almost any other, and that, of the existing genera, not a few are widely separated in their analogies from the others. But in the Palæotheres of the Eocene, which ranged in size from a large horse to a hare, not a few of the missing links have been found,—links connecting the tapirs to the hogs, and the hogs to the Palæotheres proper; and there is at least one species suggestive of an union of some of the more peculiar traits of the tapirs and the horses. It was among these extinct Pachydermata of the Paris basin that Cuvier effected his wonderful restorations, and produced those figures in outline which are now as familiar to the geologist as any of the forms of the existing animals. The London Clay and the Eocene of the Isle of Wight have also yielded numerous specimens of those pachyderms, whose identity with the Continental ones has been established by Owen; but they are more fragmentary, and their state of keeping less perfect, than those furnished by the gypsum quarries of Velay and Montmartre. In these the smaller animals occur often in a state of preservation so peculiar and partial as to excite the curiosity of even the untaught workmen. Only half the skeleton is present. The limbs and ribs of the under side are found lying in nearly their proper places; while of the limbs and ribs of the upper side usually not a trace can be detected,—even the upper side of the skull is often awanting. It-would almost seem as if some pre-Adamite butcher had divided the carcasses longitudinally, and carried away with him all the upper halves. The reading of the enigma seems to be, that when the creatures lay down and died, the gypsum in which their remains occur was soft enough to permit their under sides to sink into it, and that then gradually hardening, it kept the bones in their places; while the uncovered upper sides, exposed to the disintegrating influences, either mouldered away piecemeal, or were removed by accident. The bones of the larger animals of the basin are usually found detached; and ere they could be reconstructed into perfect skeletons, they taxed the extraordinary powers of the greatest of comparative anatomists. Rather more than twenty different species of extinct mammals have been detected in the Paris basin,—not a great number, it may be thought; and yet for so limited a locality we may deem it not a very small one, when we take into account the fact that all our native mammals of Britain and Ireland amount (according to Fleming), if we except the Cetaceæ and the seals, to but forty species.
