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 Proe'tidae, 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, Asaplius, Megalaspis, Liclias, 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 pleura! 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 



