PREFACE. 



The plates of this Decade were engraved a year ago, but the 

 pressure of other duties compelled Mr. Salter, the late Palaeontologist 

 of the Geological Survey, to postpone the descriptions. That 

 gentleman has now completed the work at my request, he being 

 at present engaged on a Monograph of all the British Trilobites for 

 the Falseontographical Society, 



Following our usual plan, those genera only have been illustrated 

 which are rich in material for the engraver, or those which, though 

 less perfect, are so rare and interesting as to render it desirable to 

 publish them, even from fragmentary materials. The genus Olenus, 

 characteristic of the lowest Silurian rocks of all the northern 

 parts of Europe, is an instance in point. All the British species^ 

 save one or two, are fragmentary, but these fragments illustrate 

 several of the important sub-genera into which this genus of the 

 primordial Silurian of Barrande is divisible ; hence a second plate 

 of it is given, ^glina, pi. IV., is another case of the same kind. 

 The genus was previously illustrated in Decade 7, from imperfect 

 pieces of the head and body. We now possess the entire form of 

 this genus, which is eminently Lower Silurian in its range. 



The genera Stygina, Salteria, Trimerocephalus, Angelina^ 

 Agnostics, and Staurocephalus have not before been illustrated in 

 the Decades. Asaphus, before given in Decade 2 as to one of its 

 sub-generic groups, now presents us with a British type of the 

 sub-genus Isotelus, which is so common in America, but rare in 

 Europe. Lastly, a fresh discovery, by the author of this Decade, of 

 a gigantic Paradoxides in the Lingula flags of Pembrokeshire, has 

 made it possible to figure this characteristic genus from perfect 

 materials. 



KODERICK I. MURCHISON, 



DiTectoT-General. 



Geological Survey Office, 

 28, Jermyn Street, London, 

 November 1864. 



[XI.] 



