222 TRILOBITA CHAP. 
period the group must have been of considerable antiquity ; but 
of its pre-Cambrian ancestors nothing is yet known; consequently 
there is no direct evidence of the origin of the group. 
Trilobites form an important part of all the faunas of the 
Cambrian system; they attain their greatest development in the 
Ordovician period, after which they become less numerous; their 
decline is very marked in the Devonian, in which nearly all the 
genera are but survivals from the Silurian period; in the 
Carboniferous, evidence of approaching extinction is seen in 
the small number of genera represented, all of which belong to 
one family—the Proétidae, in the relatively few species in each 
genus and in the small size of the individuals of those species. 
In Europe no representatives of the group appear to have 
survived the Carboniferous period, but in America one form has 
been recorded from deposits of Permian age. 
Trilobites seem to have been exclusively marine, since they 
are found only in association with the remains of marine 
animals. Their range in depth was evidently considerable, for 
they occur in many different kinds of sediment, and were 
apparently able to live regardless of the nature of the sea-floor 
—whether muddy, sandy, calcareous, or rocky. In some cases 
they occur in deposits containing reef-building corals and other 
shallow water animals; in others they are associated with 
organisms which lived at greater depths. The group appears to 
have had a world-wide distribution, for the remains of Trilobites 
are found in the Palaeozoic rocks of all countries. Their range 
in size is considerable; for whilst a large proportion of the 
species are about two or three inches in length, some, like 
Agnostus, are only a quarter of an inch long, others are 
from ten to twenty inches long, the largest forms includ- 
ing species of Paradoxides, Asaphus, Megalaspis, Lichas, and 
Homalonotus. 
The feature in a Trilobite which first attracts attention is 
the marked division of the dorso-ventrally flattened body into a 
median or axial part, and a lateral or pleural part on each side. 
It was this character that led Walch, in 1771, to give the name 
by which the group is now known. The axial part of the body 
contained the alimentary canal, as is shown by the position of 
the mouth and anus, as well as by casts in mud of the canal 
which are found in some specimens. The trilobation of the 
