278 Alfred Sherwood Romer 



are closely associated with the eurypterids and fishes. Resort may be had to the 

 supposition that the fauna was originally a typically marine one, and that diagenetic 

 processes have removed most of the invertebrates. There has been a process of solution 

 of carbonate shells in at least certain instances in the Ki Zone, but the known inverte- 

 brate forms are adequately identifiable from the moulds remaining; considering the 

 long series of students of these beds, had a richer marine fauna been present, indica- 

 tions of it would surely have been discovered and described. Here, as in other 

 instances, I find myself sceptical — reasonably, I think — of hypotheses which propose 

 diagenetic disappearance of marine invertebrates exclusively in those beds of a series 

 which carry fish. 



On the whole, the evidence suggests that the eurypterid-fish beds of Ki are deposits 

 laid down in lagoons in which on the average the water was of low salinity. With 

 this much, I think most students of the subject would agree. From this point onwards 

 advocates of fresh- or salt-water fish origins may reasonably disagree. (1) It may be 

 assumed by those who favour marine origins that the lagoonal conditions were 

 uniformily brackish during the time of deposition of the Ki beds and that the fishes 

 were marine forms but euryhaline, tolerant enough of brackish conditions to exist 

 here as well as in a normal marine environment. (2) Again assuming uniformity of 

 brackish conditions, fresh water advocates may argue that the fishes of the time were 

 normally inland dwellers but sufficiently euryhaline to descend to brackish waters. 

 (3) A third possibility is that the lagoons were deltaic and fluctuated from time to time 

 in salinity, with the fauna shifting from time to time between one of fish and euryp- 

 terids when the waters were relatively fresh, and one with sparse invertebrates replacing 

 them when the salt content increased. 



All three assumptions are equally reasonable as far as the evidence from Oesel 

 alone is concerned. I am strongly disinclined to accept the first hypothesis and tend 

 to favour the second, and more especially the third; this not alone because of my 

 admitted prejudice, but particularly because the complete lack, as we have seen, of 

 any comparable fauna of fish of this age in any typical marine bed. 



CONCLUSIONS 



I have above reviewed all fish occurrences from the European Silurian (and Down- 

 tonian) cited by Gross; these include all described pre-Devonian vertebrate finds 

 except for those from the relatively few American continental localities previously 

 studied and a few scattered European finds omitted by Gross or described too late 

 to be included in his review. They may be grouped under the following headings : 



(1) From typical marine deposits (as in England, Podolia, Bohemia) a very few 

 rare and generally fragmentary remains, mainly scales or spines, which may most 

 reasonably be regarded as strays. 



(2) Finds in beds of Downtonian or " Old Red " facies of transitional or continental 

 type, as in Great Britain, Norway, Podolia, Bohemia and Pas-de-Calais. 



(3) Isolated scales and spines from late Silurian Baltic deposits (as the Beyrichien- 

 kalk, Scania, Zones K2-K4 of Oesel) which are of somewhat questionable nature but 

 on the whole reasonably interpreted as materials drifted out from the advancing 

 shore-line. 



