he ancient type, or long-extinct molluscs; while reptiles equally
gigantic, and of still stranger proportions, haunted the neighboring
swamps and savannahs; and when the same sun that shone on the tall
moving forms beside the waters, and threw their long shadows across the
red sands, lighted up the glades of deep forests, all of whose fantastic
productions,--tree, bush, and herb,--have even in their very species
long since passed away. And of this scene of things only the footprints
remain,--"footprints on the sands of time," that tell us, among other
matters, whence the graceful American poet derived his quiet but
singularly effective and unmistakeably indigenous figure:--

"Lives of great men all remind us
We can make our lives sublime,
_And, departing, leave behind us_
_Footprints on the sand of time._
Footprints that perhaps another,
Sailing o'er life's solemn main,
A forlorn and shipwrecked brother,
Seeing, shall take heart again."

[Illustration: Fig. 69.

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 interm