Fish origins— fresh or salt water "^ 273 



invertebrate fauna from the Czortkow as a whole, but we should not jump to ihc 

 conclusion that the fishes are intimately associated with this fauna. To cite (in trans- 

 lation) KozLOwsKi's description (1929, 14) of the Czortkow, in part: " In the lower 

 beds . . . the fauna is composed mainly of brachiopods, teniaculitcs, little ostracods 

 and pelecypods. In the upper beds . . . the lentaculites and the brachiopods diminish 

 progressively in number, while the leperditiids and the tishcs become more and more 

 numerous. Finally, in the terminal beds ... the brachiopods are very rare (with the 

 exception of lingulas), the fauna being principally made up of ostracods, among which 

 the leperditiids dominate, filling entire beds, and of fishes; here one also finds frag- 

 ments of Pterygotus. This difference between the fauna of the lower part of the stage 

 and that of its upper part has been remarked on by most of the authors occupying 

 themselves with the Silurian of Podolia. . . . However, the passage from one fauna to 

 the other occurs in a gradual manner, and certain species characteristic of the first 

 continue in the second until its disappearance. . . . This underlines the unity of the 

 Czortkow fauna from its commencement until the moment when continental con- 

 ditions have invaded the marine basin and put an end to its existence."" 



KozLOWSKi further states, in litteris: 



" Dans I'etage de Czortkow, en allant de sa base vers le sommet, on constate un 

 developpement de plus en plus accentue du facies schisto-greseux. Ce que Brotzen 

 appelle ' Uebergangsschichten " et ' Schichten mit griinen Sandsteinen " correspond 

 a la partie superieure de la serie de Czortkow qui est separee de sa partie moyenne par 

 un epais banc de gres micace. A partir de ce banc les gres et schistes dominent de 

 plus en plus sur les calcaires a faune marine, ces derniers ne formant que de minces 

 intercalations dans la serie schisto-greseuse. II me semble qu"on y a a faire a un 

 empietement de plus en plus accentue du milieu continental sur le milieu marin. Et 

 il est significatif qu' a ceci correspond un enrichissement progressif des sediments en 

 restes de Vertebres, surtout de Pteraspides. Aujourd'hui— toutes mes notes ayant 

 disparu pendant le guerre— je ne saurais dire si ces Vertebres se rencontrent seulement 

 dans les couches de schistes et de gres ou s'ils se presentent egalement dans les 

 intercalations calcaires a faune marine. Quoiqu'il en soit on a Timpression que leur 

 presence dans le serie de Czortkow est essentiellement liee aux avancements periodiques 

 des depots deltaiques dans le domaine marin. On y observe aussi que dans les inter- 

 calations marines, en allant du bas vers le haut de la serie, les Brachiopodes sont 

 progressivement remplaces par les Lamellibranches et surtout par les Ostracodes." 



No extensive comment is, I think, needed. We have here an excellent example, in 

 an area towards and over which continental Old Red deposition was advancing, of a 

 transition from typical marine beds to brackish, presumably estuarine deposits, and to 

 continental redbeds, much as in England or Norway. As in those countries, fishes 

 appear in numbers only as continental conditions are approached. Far from indicating 

 a marine fish habitat, the Podolian situation strongly supports the thesis that Iresh 

 waters were the centre of Silurian vertebrate life. 



4. Bohemia . , . • 



We are here beyond the limits of the Downtonian advance of continental deposits 

 from the north, but it appears that although much of Bohemia was covered by seas 

 during the Silurian, there was present here an island area (ct. Fig. 3) which was the 

 nucleus from which a larger land mass emerged in Devonian times. The Bohemian 



