numbers, but of a very different pattern
from the Oysters, Clams, and Mussels of recent times. The annexed
wood-cut represents one of these Brachiopods, which form a very
characteristic type of the Silurian deposits. The square cut of the
upper edge, where the two valves meet along the back and are united by
a hinge, is altogether old-fashioned, and unknown among our modern
Bivalves. The wood-cut does not show the inequality of the two valves,
also a very characteristic feature of this group,--one valve being flat
and fitting closely into the other, which is more spreading and much
fuller. These, also, were represented by a great variety of species, and
we find them crowded together as closely in the ancient rocks as Oysters
or Clams or Mussels on any of our modern shores. Besides these, there
were the Bryozoa, a small kind of Mollusk allied to the Clams, and very
busy then in the ancient Coral work. They grew in communities, and the
separate individuals are so minute that a Bryozoan stock looks like
some delicate moss. They still have their place among the Reef-Building
Corals, but play an insignificant part in comparison with that of their
predecessors.

Of the Silurian Univalves or Gasteropods there is not much to tell, for
their spiral shells were so brittle that scarcely any perfect specimens
are known, though their broken remains are found in such quantities as
to show that this class also was very fully represented in the earliest
creation. But the highest class of Mollusks, the Cephalopods or
Chambered Shells, or Cuttle-Fishes, as they are called when the animal
is unprotected by a shell, are, on the contrary, very well preserved,
and they are very numerous. Of these I will speak somewhat more in
detail, because their geological history is a very curious one.

[Illustration]

The Chambered Nautilus is familiar to all, since, from the exquisite
beauty of its shell, it is especially sought for by conchologists;
but it is nevertheless not so common in our days as the Squids and
Cuttle-Fishes, which are the most numerous modern representatives of the
class. In the earliest geological days, on the contrary, those with a
shell predominated, differing from the later ones, however, in having
the shell perfectly straight instead of curved, though its internal
structure was the same as it is now and has ever been. Then, as now, the
animal shut himself out from his last year's home, building his annual
wall behind him,