CRUSTACEA. TRILOBITA 311 



antiquity, but at present no traces of the ancestors of the 

 Cambrian forms have been found. It is in the Cambrian 

 System that we meet with the largest, as well as the 

 smallest Trilobites, e.g. Paradoxides and Agnostus. As a 

 whole, it may be said that the Trilobites which are con- 

 fined to the Cambrian period are characterised by the 

 possession of a large number of thoracic segments, and of 

 a small pygidium (fig. 130) ; whereas, in the Ordovician, 

 most of the characteristic genera have fewer segments in 

 the thorax and possess large pygidia (fig. 132). 



Many of the Trilobites seem to have had a wide 

 geographical distribution; for example, most of the genera 

 which have been recognised in Australia occur also in 

 Europe. Some, however, apparently had a more limited 

 range; thus, for instance, Sao, Arethusina, and Ellipso- 

 cephalus are very common in Bohemia, but are seldom 

 found elsewhere. 



The most important genera found in the different 

 systems are mentioned below ; those marked with an 

 asterisk* occur only in one system. 



Cambrian. Agnostics, Microdiscus* ', Paradoxides*, Olenellus*, 

 Sao*, Ellipsocephalus* , Concoryphe*, Olemcs*, Niobe, Angelina*. 



Ordovician. Agnostics, Ampyx, Trinucleus* , Ogygia, Asaphus, 

 Illomics, JEglina*, Calymene, Cybele*, Lichas. Ogygia, Asaphics, 

 Triniccleus and Ampyx are abundant. 



Silurian. Calymene, Homalonotics, Illamus, Phacops, Cheimcrus, 

 Deiphon*, Sphwrexochus, Encrinurus, Acidaspis, Proetus, Lichas. 

 Calymene and Phacops are particularly abundant. 



Devonian. Homalonotics, Bronteics, Phacops, Cheirurus, Proetics. 



Carboniferous. Phillipsia, Grij/ithides*, Brachymetopus*. 



