2 BRITISH DEVONIAN BRACHIOPODA. 



formations. So exceedingly disturbed are the strata, that without a new survey it is to 

 this day impossible to say what unconformities may or may not exist among its members. 

 Neither, till the country is remapped, is it possible to make out accurately the exact 

 zoological relations of the subdivisions ; and the data I now present are only approximate, 

 being the result of an examination of the Devonian lists with Mr. Salter, who placed his 

 intimate knowledge of the ranges of Palaeozoic forms at my disposal. 



" First, then, the Devonian fossils are distinct from those of the Silurian rocks of the 

 district, a circumstance easily accounted for when we know that they lie directly and 

 unconformably on Lower Silurian strata. 



"This, therefore, makes it impossible to prove that in Devon or Cornwall the lowest 

 Devonian rocks exist. In North America, where such beds lie directly on Upper Silurian 

 strata, it is plain that the latter suffered extensive erosion before the deposition of the 

 former, this physical break being accompanied by a marked break in organic succession. 

 There is thus reason to believe that, if our Upper Silurian strata were in contact with the 

 Lower Devonian rocks of Devon and Cornwall, the same broken relations would exist 

 between them ; for of the several hundreds of Upper Silurian forms, it is said that only 

 about six species occur in the Lower Devonian rocks, perhaps not more than 1 per cent. 



" Thus, therefore, we have a zoological break, all but total, between two formations, 

 accompanied, probably, by a vast lapse of time unrepresented by any strata in Devonshire, 

 and only possibly represented by the so-called unfossiliferous Lower Old Red Sandstone of 

 Ireland, Scotland, and Wales. 



" There being, in Mr. Salter's opinion, no marked zoological line between our Lower 

 and Middle Devonian orders, I have massed them, and divided the series as follows : — 



" 1. Marwood and Pilton beds =the Coomhola Grits of Mr. Jukes and Carboniferous 

 Slate of Sir R. Griffith. 



"2. Upper Devonian strata. 



" 3. Middle and Lower Devonian. 



" The result of an elaborate analysis from tables prepared for the purpose, is that of 

 known Middle and Lower Devonian fossils there are about Gl genera and 170 species; 

 and of these about 23 species only pass into the Upper Devonian division, or about 1 3!j 

 per cent. 



"The Upper Devonian beds yield about 30 genera and 70 species; and of these 24 

 (or about 34 per cent.) pass into the Marwood beds, which in Devonshire have heretofore 

 given about 31 genera and G5 species, of which 14 species, or about 21 per cent., pass 

 into the Carboniferous rocks. Judged by the imperfect data of mere per-centages, it 

 appears, then, that the Upper Devonian are less intimately connected with the Lower 

 Devonian than with the Marwood beds, and that the Marwood beds are zoologically more 

 nearly related to the Upper Devonian than to the Carboniferous strata. In the south-west 



