272 Alfred Sherwood Romer 



in Gross's paper. As he has done, we will here include the Downtonian as part of the 

 Silurian, although recognizing the growing tendency on the part of many strati- 

 graphers to include it in the Devonian. 



1 . Great Britain 



In Great Britain (Jones, 1929; King, 1934, etc.) the Silurian was for most of its 

 duration a time of marine deposition (Valentian, Wenlockian), and in correlation 

 with this there are no vertebrates recorded until fairly late in the period. The upper 

 Silurian Ludlow beds are likewise marine, but there are a few records of fragmentary 

 vertebrate remains; Enghsh stratigraphers generally agree, however, that these are 

 " strays " floated out from the shore-line of the advancing land areas at the summit 

 of the Ludlow. 



Vertebrates first appear in numbers in the famous Ludlow Bonebed, marking the 

 arrival of continental conditions and the transition to the Downtonian. This last is 

 obviously a deltaic formation. There are occasional eruptions of brackish to salt 

 waters (as indicated by the presence of Lingula, etc.), but essentially it is a fresh- water 

 deposit, with a paucity of invertebrates and a diversified fish fauna. As in many other 

 Silurian and early Devonian continental deposits, the fishes are accompanied by 

 eurypterids and ceratiocarids. The British record is thus consistent with beliefs as to 

 the general fresh water nature of early fish faunas: fishes appear only as continental 

 conditions are approached or reached. 



2. Norway 



Here again the fish record is clearly one indicating a fresh-water habitus. The 

 Norwegian Silurian is restricted in area, and found mainly in the Oslo fiord region, 

 as at Ringerike (Kiaer, 1908, 1924; Heintz, 1939). In Norway, marine Silurian 

 beds are, as in England, succeeded by continental " redbeds " comparable to the 

 English Downtonian but possibly somewhat earlier in age. In these beds typical 

 marine fossils are absent; as usual, the vertebrates are accompanied by eurypterids 

 and ceratiocarids. There are no vertebrates in the marine part of the section; fishes 

 appear when continental deposits appear. 



3. Podolia 



The late Gotlandian and " Passage Beds " of Podolia contain a considerable fauna 

 of vertebrates — about 24 types, described by Zych (1927), Brotzen (1933 a, 1936), 

 and Stensio (1944) — which are similar to those of the continental Downtonian. 

 However, there is a considerable marine fauna in the Podolian beds as a whole, and 

 hence these forms are classed by Gross (46-49) as marine. 



Let us, however, examine more closely the stratigraphic situation as described by 

 KozLOWSKi (1929, 1-23; more recently summarized by Samsonowicz, 1950, 499- 

 503). KozLOWSKi distinguishes three successive Silurian stages. The lowest, the Skala 

 stage, has a good marine invertebrate fauna; no vertebrates are present. In the second, 

 Borszczow, stage there is likewise a good marine fauna. There are no reports of verte- 

 brates except that Brotzen (1936) mentions that indeterminate acanthodian scales 

 have been found. The fish remains are thus almost entirely confined to a final stage, 

 the Czortkow, which is many ways comparable to the Downtonian, and is succeeded 

 by typical continental Old Red deposits. Kozlowski cites a considerable marine 



