456 '' Geological Society, 



2. ' The Base of the Devonian, with especial reference io the 

 "Welsh Borderland.' By Laurence Dudley Stamp, D.Sc, A.K.C., 

 F.G.S. 



This paper is an attempt to establish a satisfactory base for the 

 Devonian System, on a basis which shall serve for intornutionul 

 correlation. The classification adopted is as follows : — 



Lower Devonian (Old Red/ II. Dittonian. 

 Sandstone Facies.) \ I. Downtonian. 



{III. Liidlovian. 

 II. Wenlockian. 

 I. Valentian. 



Murchison, in defining the Silurian System, drew the limit 

 between it and the Old Bed Sandstone below the Dowuton Castle 

 Sandstone or Tilestones. Ho afterwards included the latter in 

 the Silurian, and later writers have groui)ed still higher beds 

 as Silurian. It is pr()j)osed to return to the original definition of 

 Murchison. The Ludlow Bone-bed forms a natural base : it con- 

 sists of fish-remains, all of which first api)ear at this hoi'izon, and 

 are genetically connected with higher Devonian faunas ; it passes 

 laterally into a conglomei-ate, and thus forms a natural physical 

 base ; it marks a palyeontological and lithological break which can 

 be correlated all over North- Western Europe. Typical sections are 

 described in detail. 



A stud}' of the paheontology of the Downtonian of the Welsh 

 Borderland shows that the fauna of the lower beds (Ludlow Bone- 

 bed, Downton-Castle Sandstone, and Plafi/schisma Shales) falls 

 into three groups : — 



(rt) Upper Ludlovian marine species which survived the change 

 of conditions indicated by the bone-bed, and lived on in diminished 

 numbers, but gradually die out. The I'la/j/scJiismft Shales of Clun 

 Forest are of deeper-water type than the Downton-Castle Sand- 

 stone, and still other marine forms occur. 



(i) Species which flourished for a short time under the changing 

 conditions. 



(c) New forms — chicfl}' fishes — which persist, or are closely 

 connected with later Devonian forms. 



A comparison is made between the succession in the Welsli 

 Borderland and various other regions : — Scotland, Devon and 

 Cornwall, Northern France, the Ardennes, Britanny, Portugal, the 

 Baltic lands, Spitsbei^gen, and North America. 



Some notes on the Downtonian paheogcography of England are 

 added, and some remarks on the liabitat of Devonian fishes. It 

 is suggested, from the association of the early Downtonian fishes 

 with marine invertebrates that the}' could live in either salt or 

 brackish water, but gradually became s])ecialized — some for a 

 lacustrine habitat, others, perhaps, for marine conditions. 



