BTHERIDGE BEVONIAN ROCKS AND FOSSILS. 663 



It is well known that in the Continental areas where the Devonian 

 system is well developed, and also in Britain, certain and pecu- 

 liar genera occur which are represented by a limited number of 

 species ; yet the fauna of the Devonian period, so far as at present 

 known, when compared with those of the two periods (the Silurian 

 and Carboniferous), is comparatively poor *. 



We may assume that eleven classes are represented in the fauna 

 of the Devonian system, viz. the Amorphozoa, Coelenterata, Echino- 

 dermata, Crustacea, Polyzoa, Brachiopoda, Lamellibranchiata, Nu- 

 cleobranchiata. Gasteropoda, Cephalopoda, and Pisces, — the last but 

 sparingly (as far as we know), but still of considerable importance 

 as bearing upon the question of the synchronism of occurrence of 

 certain forms of life in deposits believed to bo of equal value, or 

 equivalent in time, though accumulated under different conditions 

 and in diiferent areas. We know that 9 of the 11 classes do not 

 occur in the true Old Eed Sandstone, none of the marine De- 

 vonian MoUusca, Coelenterata, and Echinodermata having any re- 

 presentative in the Scotch, Welsh, or English areas occupied by that 

 group of rocks ; but evidence of late tends to connect these two 

 disconnected yet contemporaneous deposits through the group of 

 Fishes, so that chronologically the difficulty of correlating them is 

 partly overcome ; for the Middle and Upper Old Bed Sandstone of 

 Eussia contains a marine MoUuscan fauna associated with the Pish ; 

 Scotland contains the same Pish, but no intermixture of associated 

 and marine Invertebrata. This fact clearly identifies the Middle and 

 Upper Old Bed Sandstone of the northern or Scotch area with the 

 slates and calcareous series of the Eifel, and with the Devonshire De- 

 voniansf ; and now we have in South and IN^orth Devon conclusive 

 evidence of the same fact, from the occurrence of Pliyllolepis con- 

 centricus and Ichthyodorulites of Onchi in the Lower Devonian Slates 

 of Looe Island in South Devon, and Holoptycliiiis in the Lower and 

 Upper Devonians. 



No value is attached to the occurrence of many species of Pish 

 that occur in common at the top of the Upper Silurian series and in the 

 lower beds of the Old Bed Sandstone ; they are locally connecting 

 species, in the passage-beds, and do not occur higher ; and the want 

 of absolute chronological and paloeontological identity between the 

 Lower Old Bed Sandstone and Lower Devonian prevents our true 

 correlation of the two series. Auclienaspis SaUeri, Cephalafpis 

 Murchisoni, C. ornatus, OncJius Murcliisoni, 0. temmtriatus, Plec- 

 trodus inintbUis, P. pleiopristis, Pteraspis BanJcsu, and P. tnmcatus 

 are all Upper-Silurian species, four of which, A. Sdterl, C. Mur- 

 chisoni, C. oniahis, and Pteraspis Bcinksii, are common to the Upper 

 Silurian and Old Bed Sandstone J. The remaining 108 species of Pish 

 are distributed through the Lower, Middle and Upper Scotch Old 



* Jlde numericjil or census-table of the number of genera and species in the 

 Silurian, Old Red Sandstone, Devonian, and Carboniferous strata, p. 015. 



t Siluria, Hrd edit. pp. 382 &c. 



I For much valuable information and tabular results see Pengelly, Erit. 

 Assoc. Report, 1860, Oxford Meeting. Also Pengelly '• On the Devonian Age of 



