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_.
Pages:
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161