212 PALAEONTOLOGY 



similarly for the Middle Cambrian, and species of Olenidcz 

 for the Upper Cambrian (see Appendix I.). In the 

 succeeding Ordovician system the trilobites attain their 

 acme. Although with the modern narrowing of the 

 extent of the term " family " it is no longer possible to 

 say that all families of trilobites occur in the Ordovician, 

 yet there are few that do not, and there is a greater 

 abundance and variety of forms than in any other system. 



Two families are especially characteristic of the Ordo- 

 vician Trinucleids and Asaphida. Both are strictly 

 confined to the Ordovician, if we include the Tremadoc 

 beds, which form a transition from Cambrian to Ordo- 

 vician, and if we confine ourselves to Britain and Scan- 

 dinavia ; but asaphids have been found as low as 

 Middle Cambrian in British Columbia. They thus 

 afford an instance of migration as an explanation of 

 cryptogenetic types. Asaphids are more character- 

 istic of 'the Lower and Middle Ordovician, trinucleids 

 of the Upper. Neither family survives the period, but 

 allies of each persist into the Silurian Illcenus repre- 

 senting the asaphids and Ampyx the trinucleids. 



In the Silurian the trilobites are still abundant, but no 

 new families appear and many genera are extinct, 

 especially among Opisthoparia, the Proparia now in- 

 creasing in importance. In the Devonian the list of 

 families is diminished by three the two mentioned above 

 as surviving into the Silurian and the Encrinuridce. Lastly 

 in the Carboniferous only one family survives, the 

 Proetida, represented by four genera, of which one 

 survives into the Permian of tropical regions. 



