lder and deeper lying beds, we see
their places taken by other shells, of species altogether distinct. The
very common shell _Purpura lapillus_, for instance, is found in our
raised beaches, in our Clyde beds, in our boulder clays and
mammaliferous crags, and, finally, in the Red Crag, beyond which it
fails to appear. And such also is the history of the common edible
mussel and common periwinkle; whereas the common edible cockle, and
common edible pecten (_P. opercularis_) occur not only in all these
successive beds, but in the Coral Crag also. They are older by a whole
deposit than their present contemporaries, the mussel and periwinkle;
and these, in turn, seem of older standing than shells such as _Murex
erinaceus_, that has not been traced beyond the times of the
mammaliferous crag, or than shells such as _Scrobicularia piperata_,
that has not been detected in more ancient deposits than raised sea
beaches of the later periods, and the elevated bottoms of old estuaries
and lagoons. We thus know, that in certain periods, nearer or more
remote, all our existing molluscs _began_ to exist, and that they had no
existence during the previous periods; which were, however, richer in
animals of the same great molluscan group than the present time. Our
British group of recent marine shells falls somewhat short of _four_
hundred species;[19] whereas the group characteristic of the older
Miocene deposits, largely developed in those districts of France which
border on the Bay of Biscay, and more sparingly in the south of England,
near Yarmouth, comprises more than six hundred species. Nearly an equal
number of still older shells have been detected in a single deposit of
the Paris basin,--the _Calcaire grossier_; and a good many more in a
more ancient formation still, the London Clay. On entering the Chalk, we
find a yet older group of shells, wholly unlike any of the preceding
ones; and in the Oolite and Lias yet other and different groups. And
thus group preceded group throughout all the Tertiary, Secondary, and
Palaeozoic periods; some of them remarkable for the number of species
which they contained, others for the profuse abundance of their
individual specimens, until, deep in the rocks at the base of the
Silurian system, we detect what seems to be the primordial group,
beneath which only a single animal organism is known to occur,--the
_Oldhamia antiqua_,--a plant-like zoophyte, akin apparently to some of
our recent sertulari