614 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Old Red Sandstone of the adjoining districts of Grloucester, Hereford, 

 and South Wales, as well as Scotland, it is certain that they are most 

 intimately related to theEuroiDean series in every particnlar; and if 

 it were possible to reconstruct the old geographical area and coast- 

 lines against which, and the areas over which, the marine equiva- 

 lents of the Old lied Sandstone were deposited or accumulated, 

 we should, I think, find that the northern boundary to the British 

 marine type would be about the latitude of our Mendip Hills, 

 ranging eastwards to Coblentz and the Hundsriick, and westward 

 to the south-west of Ireland, and a region now covered by the At- 

 lantic. The southern boundary may be the north coast of France, 

 or even Asturias and the Pyrenees on the north of Spain, thus 

 constituting ah area of great extent *. 



1. Chronological Equivalents of the Old Red Sandstone of Here- 

 ford, Scotland, ^x.- — The palseontological break between the Silurian 

 rocks and the Old Eed Sandstone was nearly complete ; and although 

 they are stratigraphically conform^able (in tlie typical Silurian area), 

 yet there are only 13 species known as common to the Old Red 

 of the Silurian area and the Silurian rocks. Those species that 

 occur are represented by the Pishes and Poecilopod Crustacea, 

 which are found through the passage-beds, at all times a doubtful 

 horizon, especially so when no other zoological group exists for 

 comparison, which is so markedly the case at the close of the Silu- 

 rian and commencement of the Old-Red-Sandstone period. They 

 are the following : — CepTialaspis Murchisoni, C. ornatus, Auche-nasjns 

 Salteri, Onclius Murchisoni, Pteraspis BanJcsii, P. Lloydii, and P. 

 truncatus. One genus only of these (Onchus) is represented in the 

 Carboniferous series. The Crustacea, which are on the confines 

 also of the t^vo, are : — Eurypterus ahhreviatus, Salt., E. acuminatus, 

 Salt., E. pygmoius, Salt., Pterygotus anglicus, Ag., P. prohlematiciis, 

 Salt., and Sti/lonurus megalops. 



It must be remembered that there are nearly 1160 described 

 species in the Silurian rocks, and 149 (113 of which are Pish) in the 

 Old Red Sandstone (Table I.), and only the above 7 Pish and 6 Crus- 

 tacea are known to occur as common to the two formations, and 

 that strictly on their confines f. 



The accompanying Table is constructed to show the census of 

 the Palaeozoic rocks as now known, the number of species in each 

 class being numerically expressed. The fifth column shows the 

 number of species (56) common to the marine Devonian and the 

 Carboniferous rocks ; it is here incorporated as being also of value in 

 future parts of the paper. 



* This area will be more definitely noticed hereafter. 



t It is stated by Prof. Jukes (Quart. Journ. Geol. See. vol. xxii. 1866, p. 366) 

 that a few species, svich as Sti-ophomena rhomlmdalis, occur in both Silurian 

 and Carboniferous : this must be aii error ; I know no one species common to 

 these two great formations. 



