86 HISTORICAL PALZONTOLOGY. 
brian Trilobites very commonly have either a great many rings 
(as in Paradoxides, fig. 31, a), or else very few (as in Agnostus, 
fig. 31, g). In some instances, the body-rings do not seem to 
have been so constructed as to allow of much movement, but 
in other cases this region of the body is so flexible that the 
animal possessed the power of rolling itself up completely, like 
a hedgehog ; and many individuals have been permanently 
preserved as fossils in this defensive condition. Finally, the 
body of the Trilobite was completed by a tail-shield (techni- 
cally termed the “‘pygidium”), which varies much in size and 
form, and is composed of a greater or less number of rings, 
similar to those which form the thorax, but immovably amalga- 
mated with one another (fig. 31, 7). 
The under surface of the body in the Trilobites appears to 
have been more or less entirely destitute of hard structures, 
with the exception of a well-developed upper lip, in the form 
of a plate attached to the inferior side of the head-shield in 
front. ‘There is no reason to doubt that the animal possessed 
legs; but these structures seem to have resembled those of 
many living Crustaceans in being quite soft and membranous. 
This, at any rate, seems to have been generally the case ; 
though structures which have been regarded as legs have been 
detected on the under surface of one of the larger species of 
Trilobites. There is also, at present, no direct evidence that 
the Trilobites possessed the two pairs of jointed feelers (‘‘an- 
tenne”) which are so characteristic of recent Crustaceans. 
The Trilobites vary much in size, and the Cambrian forma- 
tion presents examples of both the largest and the smallest 
members of the order. Some of the young forms may be little 
bigger than a millet-seed, and some adult examples of the 
smaller species (such as Agwostus) may be only a few lines in 
length ; whilst such giants of the order as Paradoxides and 
Asaphus may reach a length of from one to two feet. Judging 
from what we actually know as to the structure of the Tnilo- 
bites, and also from analogous recent forms, it would seem that 
these ancient Crustaceans were mud-haunting creatures, deni- 
zens of shallow seas, and affecting the soft silt of the bottom 
rather than the clear water above. Whenever muddy sedi- 
ments are found in the Cambrian and Silurian formations, 
there we are tolerably sure to find Trilobites, though they are 
by no means absolutely wanting in limestones. ‘They appear 
to have crawled about upon the sea-bottom, or burrowed in the 
yielding mud, with the soft under surface directed downwards; 
and it is probable that they really derived their nutriment from 
‘the organic matter contained in the ooze amongst which they 
