Old Rea Sandstone. 103 



Megalodon is characteristic. Of the 13 genera of 

 Gasteropoda, 9 are Silurian, but of 47 species, all are 

 distinct. The most prevalent forms belong to the 

 genera Euomphalus (6), Loxonema (8), Macrocheilus 

 (7), Murchisonia (5) (there are 22 in the Silurian 

 rocks), and Pleurotomaria 8. There are 5 species of 

 Bellerophon, and 52 species of Cephalopoda, all distinct 

 from Silurian species. Of 6 Devonian genera, only 

 Orthoceras, Poterioceras, and Cyrtoceras are Silurian. 

 The most prevalent species belong to the genera Clymenia 

 (11), Cyrtoceras (13), Goniatites (10), this being their 

 first appearance in the British strata, and Orthoceras 

 (15), (there being 67 known species of this genus in the 

 Silurian rocks). 



It is stated that only about 10 per cent, of Upper 

 Silurian fossils pass into the marine Lower Devonian 

 strata. These two formations in England are, however, 

 not found in contact, though they occur commonly 

 enough in the regular order of succession on the 

 Continent and in North America. About 10 per cent, 

 of Lower Devonian fossils pass into the Middle Devonian, 

 and about the same percentage from the Middle into 

 the Upper. If this be true there may possibly be 

 undiscovered unconformities between the subdivisions. 



THE OLD EED SANDSTONE, as distinct from the 

 Devonian rocks, is undoubtedly intermediate in age to 

 the uppermost Silurian and the lowest Carboniferous 

 strata. It is sometimes difficult to determine its 

 precise limits either at its base or its top. It first 

 received its name in contradistinction to the New Red 

 Sandstone, the former occurring below, and the latter 

 above the Carboniferous strata. 



A vast triangular tract of Old Red Sandstone lies 

 between the west coast of South Pembrokeshire, Bristol 



