86 HISTORICAL PALAEONTOLOGY. 



brian Trilobites very commonly have either a great many rings 

 (as in Paradoxides, rig. 31, a), or else very few (as in Agnostus, 

 fig. 3i,'). 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, /z). 



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- 

 tennae") 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 maybe little 

 bigger than a millet-seed, and some adult examples of the 

 smaller species (such as Agnostus} 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 Trilo- 

 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 m the ooze amongst which they 



