THE CAMBRIAN PERIOD. 87 
lived. The vital organs seem to have occupied the central lobe 
of the skeleton, by which they were protected ; and a series of 
delicate leaf-like paddles, which probably 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 addi- 
tional 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 pro- 
bable has yet been suggested. If, however, the tracks which 
have been described from the Potsdam Sandstone of North 
America nnder the name of Protichnites are really due to the 
peregrinations of some Trilobite, they must have been pro- 
duced 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 (Long- 
mynd Rocks), representatives of some half-dozen genera have 
now been detected, including the dwarf Agwostus and the giant 
Paradoxidées. In the higher beds, the number both of genera 
and species is largely increased ; and from the great compara- 
tive abundance of individuals, the Trilobites have every right 
to be considered as the most characteristic fossils of the Cam- 
brian 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, Zzzguleléa, 
Discina, and Obolelia. Of these the last (fig. 32, 2) is highly 
characteristic of these ancient deposits ; whilst Dzscima 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. Lzngulella (fig. 32, c), again, is closely allied 
to the existing “‘ Goose-bill ” Lamp-shell (Zzngula anatina), and 
thus presents us with another example of an extremely long- 
