We thus find on the Cambrian rocks forty-
five thousand feet at least of newer rocks, in several cases lying
unconformably on each other, showing thereby that the lower beds had
been upheaved, and their edges worn off on a sea-shore, ere the upper
were laid down on them; and throughout this vast thickness of rocks,
the remains of hundreds of forms of animals, corals, shells, fish,
older forms dying out in the newer rocks, and new ones taking their
places in a steady succession of ever-varying forms, till those in
the upper beds have become unlike those in the lower, and all are
from the beginning more or less unlike any existing now on earth.
Whole families, indeed, disappear entirely, like the Trilobites,
which seem to have swarmed in the Silurian seas, holding the same
place there as crabs and shrimps do in our modern seas. They vanish
after the period of the coal, and their place is taken by an allied
family of Crustaceans, of which only one form (as far as I am aware)
lingers now on earth, namely, the "King Crab," or Limulus, of the
Indian Seas, a well-known animal, of which specimens may sometimes be
seen alive in English aquaria. So perished in the lapse of those
same ages, the armour-plated or "Ganoid" fish which Hugh Miller made
so justly famous--and which made him so justly famous in return--
appearing first in the upper Silurian beds, and abounding in vast
variety of strange forms in the old Red Sandstone, but gradually
disappearing from the waters of the world, till their only
representatives, as far as known, are the Lepidostei, or "Bony
Pikes," of North America; the Polypteri of the Nile and Senegal; the
Lepidosirens of the African lakes and Western rivers; the Ceratodus
or Barramundi of Queensland (the two latter of which approach
Amphibians), and one or two more fantastic forms, either rudimentary
or degraded, which have lasted on here and there in isolated stations
through long ages, comparatively unchanged while all the world is
changed around them, and their own kindred, buried like the fossil
Ceratodus of the Trias beneath thousands of feet of ancient rock,
among creatures the likes whereof are not to be found now on earth.
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