r. Leidy, of Philadelphia, has
lately described (1851) two species of cetacea of a new genus, which he
has called Priscodelphinus from the green sand of New Jersey, which
corresponds in age with the English Chalk or the cretaceous strata above
the gault. The specimens consist of dorsal and cervical vertebrae.[227]
Even in the Eocene strata of Europe, the discovery of cetaceans has
never kept pace with that of land quadrupeds. The only instance cited in
Great Britain is a species of Monodon, from the London clay, of doubtful
authenticity as to its geological position. On the other hand, the
gigantic Zeuglodon of North America occurs abundantly in the Middle
Eocene strata of Georgia and Alabama, from which as yet no bones of land
quadrupeds have been obtained.

In the present imperfect state then of our information, we can scarcely
say more than that the cetacea seem to have been scarce in the secondary
and primary periods. It is quite conceivable that when aquatic saurians,
some of them carnivorous, like the Ichthyosaurus, were swarming in the
sea, and when there were large herbivorous reptiles, like the Iguanodon,
on the land, the class of reptiles may, to a certain extent, have
superseded the cetacea, and discharged their functions in the animal
economy.

That mammalia had been created long before the epoch of the Kimmeridge
clay, is shown by the Microlestes of the Trias before alluded to, and by
the Stonesfield quadrupeds from the Inferior Oolite. And we are bound to
remember, whenever we infer the poverty of the flora or fauna of any
given period of the past, from the small number of fossils occurring in
ancient rocks, that it has been evidently no part of the plan of Nature
to hand down to us a complete or systematic record of the former history
of the animate world. We may have failed to discover a single shell,
marine or freshwater, or a single coral or bone in certain sandstones,
such as that of the valley of the Connecticut, where the footprints of
bipeds and quadrupeds abound; but such failure may have arisen, not
because the population of the land or sea was scanty at that era, but
because in general the preservation of any relics of the animals or
plants of former times is the exception to a general rule. Time so
enormous as that contemplated by the geologist may multiply exceptional
cases till they seem to constitute the rule, and so impose on the
imagination as to lead us to infer the non-existence of crea