512 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OP NOKTH AMERICA 



rence of Eurypterids and fresh-water fish as an indication of the change 

 from marine to non-marine conditions. 



The Baltic provinces of Eussia and Scandinavia have become famous 

 for their Siluric sections, particularly the ones exposed on the islands of 

 Oesel and Gotland. Considering Oesel first, two species of Eurypterus 

 have been reported, E. laticeps Schmidt, from two fairly perfect head 

 shields, and E. fischeri Eichwald, from many excellent individuals. The 

 latter is a very close relative of the American E. remipes. There is also 

 an abundance of fragments of Pterygotus osiliensis. The bed in which 

 these occur is a fine water-lime, very much like the Bertie, with un- 

 doubted marine beds above and below it, but the bed itself contains only 

 the Eurypterids just mentioned, in association with a large number of 

 scorpions, which were probably land animals, and certain merostomes, 

 classified by Woodward as Eurypterids, but now included under the 

 Synxiphosura by Zittel and others. Such forms are : Pseudoniscus aculea- 

 tus, one good specimen at least; Bunodes lunula, B. schrenckii, and B. 

 rugosus, including some fragments and a few well preserved individuals, 

 and Exapinurus schrenckii. In Gotland similar conditions are seen to 

 prevail. Although the sections in the northern and southern parts of the 

 island have been studied separately and correlations are not as yet com- 

 plete, still one important fact has stood out for the whole island : there 

 is everywhere a great break between the Lower and Middle Gotlandian 

 (Siluric). That there was at this time a retreat of the sea and a subse- 

 quent advance has been indicated in many places. In the north around 

 Visby a detailed section has been worked out by Hedstrom. The upper 

 meter of his division III of the Gotlandian consists of marl shales and 

 limestones, with Pterygotus osiliensis and Palceophonus nuncius Thorell 

 and Lindstrom as the characteristic fossils. Lindstrom has called this 

 the Pterygotus marl, and it is seen to lie just at the top of bed 111/. 

 Then follows a break and above the disconformity is bed IV, a conglom- 

 erate with water-worn gastropoda and portions of Spongiostroma holmi 

 Rothpletz. The Eurypterid beds here occupy lagoon areas between coral 

 re.efs of early Siluric age, and thus the association of Pterygotus and 

 scorpions with marine forms may well be accounted for, since the sea 

 would have occasional access to the lagoons. Munthe has described a 

 similar break in southern Gotland between the Burgsvik sandstone 

 (Lower Ludlow age) and the overlying oolite (of Aymestry age). The 

 sandstone shows cross-bedding of the eolian type and is followed by the 

 oolite, which is at most one meter thick, grading off into nothing in 

 places and which rests with a disconformity on the sandstone. Holm 



