492 



GEOLOGICAL SUCCESSION OF TRILOBITES. 



beds ; but is represented in the Lower Tertiary of Garnet Bay by 

 Eosphceroma. 1 



Trilobites. Trilobites are the most distinctive fossils of Primary 

 rocks. Like other Crustacea, they go through a metamorphosis, and 

 develop their characters as the test is shed. 2 They are mostly confined 

 to the Cambrian and Silurian strata. The only genera which exist 

 in the Carboniferous rocks are Bracliymetopus, Griffithides, and Phil- 

 lipsia. Bronteus is characteristic of the Devonian ; Cheirurus ranges 

 from the Tremadoc to the Devonian ; Harpes has a similar range ; and 

 Phacops and Homalonotus extends from the Arenig to the Devonian. 

 Dcdmannia is the only genus peculiar to the true Silurian rocks, though 

 Proetus and several genera survive to them. Thus, Acidaspis ranges 

 from the Llandeilo to the Ludlow ; Ampyx from the Tremadoc to the 

 Ludlow ; Calymene from the Arenig to the Ludlow ; Cyphapsis from 

 the Llandeilo to the Ludlow ; Eucrinurus from the Bala to the Lud- 

 low. A few genera reach no farther upward than the Wenlock rocks, 

 and are wanting in the Ludlow. Among such are Illcenus, which 

 commences in the Arenig ; and Lichas and Sphcerexochus, which com- 

 mence in the Llandeilo. Trinuneleus stops short of the Upper Lland- 

 overy, and begins with the Arenig. Among the Bala forms are 

 Amphion, Cybele, Staurocephalus, and Stygina. 



Fig. 106. Illamus. Fig. 107 Lichas. 



Fig. 108. Trinucleus. 



The Llandeilo period includes Eccoptocliile and Cyplioniscus, It 

 has, in common with the Arenig rocks, JEglina and Barrandia. 

 Peculiar to the Arenig are Dionide and Placoparia. The Tremadoc 

 trilobites include Psiloceplialus, Niobe, Neseuretus, and Angelina. 

 Ogygia commences in the Tremadoc, and ranges to the Llandeilo ; 

 Asaplws commences in the Tremadoc, and ranges to the Bala beds. 

 Several genera are common to the Lingula Flags and Menevian beds, 

 such as Paradoxides, Holoceplialina, and Anopolenus. Among Mene- 



1 See H. Woodward, Q. J. G. S., May 1879. 



2 C. G. Walcot, Bulletin of the Museum of Comparative Anatomy at Harvard 

 College, vol. viii., No. 10, 1881. Barrande, " System e Silurien de la Boheme." 

 Salter and Woodward in Palseontographical Society, &c. 



