AETHROPODA — TRILOBITES. 



85 



part of the shield, as in Ccdyrnmenc, Staurocephalus (Fig. 40 c), Gallery 

 and Phacops (Fig. 38), or they may stretch right back so as r^^i^^^^^^ 

 to include the genal angles, as in Olenus (Fig. 40 h), Para- 24. 

 doxides, Triarthrus (Fig. 39), Ogygia, Brontcus, and Acidaspis. Table-case 



25. 



Fig. 40. — Examples of Trilobites. a. HyTpoTp&ria,, Agnostus princeps, Oleni- 

 dian Age ; enlarged 2J times, b, Opisthoparia, Olenus cataractes, 

 Olenidian Age. c, Proparia, Staurocephalus Murchisoni, Wenlockian 

 Age. 



Trilobites with the latter character are called Opisthoparia 

 (back-cheeks), while those with the free cheeks in front only 

 are called Proparia (front-cheeks). 



Trilobites are found well developed in the oldest Cambrian Table-case 



rocks, being represented by Hypoparia, which soon die out, -v^all-case 

 and by Opisthoparia. Three genera of the Opisthoparian i4b. 

 Family Olenidae, Olenellns, Paradoxides, and Olenus, have 

 given their names to the Lower, Middle, and Upper Cambrian 

 Ages. With the Ordovician arise the Proparia, and the 

 Trilobita as a whole attain their acme. Through Silurian Table-case 

 and Devonian times they gradually decline in numbers and ^^^'^ 

 size, till in the Carboniferous Epoch only a single family 

 remains, of which one genus, PMllipsia, struggles on to the 

 Permian. 



The British trilobites are allied to those of Scandinavia Table-cases: 



and Eussia, rather than to those of Bohemia and the rest 

 of Europe and eastern North America. They have been 

 described mainly by J. W. Salter and H. Woodward in the 

 Monographs of the Palaeontographical Society, and many 

 specimens there fif]fured are in the national collection. 

 The large Paradoxides from the Middle Cambrian of St. 

 Davids will attract notice. Angelina Sedgwicki from the 



