BRITISH FOSSILS. 



DECADE THE SECOND 



THE second Decade of representations of British Fossils contains illus- 

 trations of the genera and species of Trilobites. 



These remarkable animals, belonging to a group of Crustacea, of 

 which we find no traces in strata formed subsequently to the Palaeozoic 

 period, were first observed in British rocks, Nevertheless, the investi- 

 gation of those species found in our country is as yet far from complete. 

 Our recorded knowledge of them is chiefly contained in the works of 

 Sir Roderick Murchison and of Lieut. -Colonel Portlock. In the 

 " Silurian System," only are figures to be found of our commoner spe- 

 cies, and in the " Report on the Geology of Londonderry, Tyrone, and 

 Fermanagh," is the only systematic dissertation on their arrangement, 

 which has yet been published in Britain. 



During the progress of the Geological Survey through the Silurian 

 districts of Wales and the bordering counties, much new and import- 

 ant material has been collected towards the elucidation of the structure, 

 affinities, and distribution of Trilobites. The following figures and de- 

 scriptions consequently contain much that has not been before noticed 

 respecting these animals, and new light will be found thrown on several 

 features of their organization. 



The Trilobites selected as subjects for this Decade are species of the 

 genera Phacops, Hlcenus, Asaphus, Ogygia, Calymene, and Ampyx. 

 Some of them are renowned and characteristic forms, such as Phacops 

 (Dalmannia) caudatus, Illcenus (Bumastus) Barriensis, Asaphus ty- 

 rannus, and Oyyyia Buchii, which, whilst they furnish excellent typical 

 figures illustrating generic or sub-generic groups, acquire an additional 

 [ii.] b 



