y served as respiratory organs, would appear
to have been carried on the under surface of the thorax. That
they had their enemies may be regarded as certain; but we have
no evidence that they were furnished with any offensive weapons,
or, indeed, with any means of defence beyond their hard crust,
and the power, possessed by so many of them, of rolling themselves
into a ball. An additional proof of the fact that they for the
most part crawled along the sea-bottom is found in the occurrence
of tracks and markings of various kinds, which can hardly be
ascribed to any other creatures with any show of probability.
That this is the true nature of some of the markings in question
cannot be doubted at all; and in other cases no explanation so
probable has yet been suggested. If, however, the tracks which have
been described from the Potsdam Sandstone of North America under
the name of _Protichnites_ are really due to the peregrinations
of some Trilobite, they must have been produced by one of the
largest examples of the order.

As already said, the Cambrian Rocks are very rich in the remains
of Trilobites. In the lowest beds of the series (Longmynd Rocks),
representatives of some half-dozen genera have now been detected,
including the dwarf _Agnostus_ and the giant _Paradoxides_. In
the higher beds, the number both of genera and species is largely
increased; and from the great comparative abundance of individuals,
the Trilobites have every right to be considered as the most
characteristic fossils of the Cambrian period,--the more so as
the Cambrian species belong to peculiar types, which, for the
most part, died out before the commencement of the Silurian epoch.

All the remaining Cambrian fossils which demand any notice here
are members of one or other division of the great class of the
_Mollusca_, or "Shell-fish" properly so called. In the Lower
Cambrian Rocks the Lamp-shells (_Brachiopoda_) are the principal
or sole representatives of the class, and appear chiefly in three
interesting and important types--namely, _Lingulella, Discina,_
and _Obolella_. Of these the last (fig. 32, i) is highly
characteristic of these ancient deposits; whilst _Discina_ is
one of those remarkable persistent types which, commencing at
this early period, has continued to be represented by varying
forms through all the intervening geological formations up to the
present day. _Lingulella_ (fig. 32, c), again, is closely allied
to the existing "G