BRITISH FOSSILS. 



DECADE THE SEVENTH. 



This Decade is devoted to figures and descriptions of Trilobites, a 

 group of extinct Crustacea of the highest geological interest. These 

 remarkable fossils are wholly restricted to Palaeozoic formations. 

 The progress of research has shown that the various genera and 

 species of Trilobites are remarkably characteristic of well-defined 

 geological horizons ; consequently, the study and exact definition of 

 them is laid much stress upon by the geologist whose labours are 

 directed to the investigation of the more ancient rocks. 



The recent publication of a beautiful work by M. Barrande, on 

 the Trilobites of Bohemia, in which the species are fully illustrated 

 and described, afibrds means of comparison with the specimens of 

 British Trilobites (usually less perfectly preserved), such as we did 

 not before possess. It will be seen from the following descriptions 

 that but few of our species are identical with those of Bohemia, and 

 thus we get at an interesting indication of a geographical distribu- 

 tion of these primseval animals. 



Of forty-five species here described, but one, a Phacops, — a member 

 of a different section from that previously illustrated, belongs to 

 any genus as yet selected for these Decades. 



Cheirurus is exemplified by a species heretofore known only in a 

 fragmentary state. 



Sphcerexochus mirus is a cosmopolitan fossil, of which excellent 

 specimens have been lent to us for illustration. 



Encrinurus and Acidaspis are typified by new species from the 

 lowest fossiliferous deposits. 



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