One of the uppermost divisions of the Silurian system of the state of New York and western Canada, the Waterlime group, is characterized by a peculiar fauna of large crustaceans, Eurypterus, Pterygotus, Ceratiocaris, etc. It has already been said by Sir Rhoderick Murchison that this fauna shows a great resemblance to similar crustacean faunas of the uppermost Silurian strata of Great Britain, the shales of Lesmahago in Lanarkshire and in some places near Ludlow, where the crustaceans are associated with a small Lingula, the characteristic Platyschisma helicites, and divers fish remains.
But still greater seems to be the resemblance of the American Waterlime fauna to our Eurypterus beds of the island of Oesel, in the eastern Baltic, because the most characteristic forms of both localities are two very nearly allied species of Eurypterus—the E. remipes of America and the E. fischeri, Eich., with us. Besides the Eurypterus, we have a large Pterygotus, the P. osiliensis (aff. P. bilobus, Salt.), two species of Bunodes, Eich. (connected with the English Hemiaspis), and a large Ceratiocaris, the C. nötlingi, similar to the C. maccoyanus of America.
Last summer a local collector, Mr. Simonsohn, of Wenden, in Livonia, found the metastoma of the genus Dolichopterus, hitherto only known from the American Waterlime; and so the resemblance between the American and Russian eurypterids becomes greater.
The most famous locality of our Eurypterus beds is Rootzikull, near Kielkond. Here, besides the crustaceans, we have also found fish remains—two cephalaspidean genera, Thyestes, Eich., and Tremataspis, described some years ago by Eichwald, Pander and myself. Now we have better specimens, which will be described soon by Dr. I. Rohon, of St. Petersburg, who has also lately described the first real fish remains of the Lower Silurian, from the greensand at the base of the Silurian, at Wessiks. These Eurypterus beds, consisting mostly of yellow dolomitic flagstones, are overlain by thin marly deposits, only a few inches thick, filled with small specimens of Leperditia (L. angelini), Platyschisma helicites, Sow., and small scales of fishes mostly belonging to the genus Cælolepis of Pander.
With us the Eurypterus horizon forms the base of our uppermost Silurian stage, K, according to my arrangement of our Russian Baltic Silurian in Estonia and the island of Oesel, and can be followed all over the island, from west to east, at the boundary line between the stages J and K, the former corresponding to the Wenlock of England and the Niagara limestone of North America.
The Eurypterus beds are overlain by a yellow limestone or dolomite containing Stromatopora, Favosites, Syringopora reticulata, Labechia conferta, and other corals (but not Halysites, which is restricted entirely to lower horizons of the Upper Silurian), besides Murchisonia cingulata and allied forms, Orthoceras imbricatum, O. angulatum, and O. gigantea, Ilionia prisca, Megalonus gothlandicus, Meristella didyma, Leperditia grandis, and other fossils. In the southern and southwestern portions of Oesel there follows a band of gray limestone with Atrypa prunum, Spirifer eleratus, Chonetes striatella, numerous specimens of Tentaculites and Beyrichia, peculiar forms of Calymene and Proetus, and in some places with a profusion of spines (Onchus) and scales (Tachylepis, Pand., or Ghelodus, Murch., Oniscolepis) of fishes described by Pander in 1856. This gray limestone, which is known among the northern German erratic bowlders as the Beyrichia limestone, I regard as the highest beds of Oesel, though actual superposition has not been observed. Both the gray and the yellow limestones correspond very well with the Ludlow of Great Britain. The yellow limestone containing also Eurypterus fischeri is very clearly recognized on the eastern side of the Swedish island of Gothland, near Oestergarn, and also on the Dniester in Podolia (southern Russia), from which locality the Eurypterus fischeri was originally described.
With regard to my Silurian country of Oesel, I have no reason to enter into the Hercynian question, because, as already stated, our uppermost Silurian strata correspond exactly to the typical Ludlow of England. Our Silurian is unconformably covered by the middle Devonian ("Old Red sandstone"), since in the east the Cambrian and lower Silurian strata are situated directly below the "Old Red sandstone," just as in the west they lie below the upper Silurian deposits.
The purpose of this communication is to attract the attention of American geologists to the striking resemblance of the fauna of our Baltic Eurpterus beds to the Waterlime fauna of North America, and to express the hope that our cephalaspidean fishes, or something like them, would be some time found in this country.
In coming to America it was my wish to become more intimately acquainted with the different Silurian stages, and especially with those adjacent to the Waterlime group, i.e., with the Onondaga and Guelph limestones on the one side and the Tentaculite limestone on the other. It would perhaps be possible to find other connecting links in the development of life in both countries.
Lately I have had the opportunity of seeing the Waterlime and the Tentaculite limestone at Oriskany falls in the state of New York. Both deposits together correspond very well to our uppermost eastern Baltic stage K. But, beyond this striking resemblance of the Waterlime crustacean fauna and that of our Eurypterus beds, I cannot yet compare strictly the other deposits of my uppermost Silurian zone in this country. That will perhaps be possible after returning from our long excursion, when I shall have perhaps the opportunity of seeing more of the Silurian strata in the United States and Canada.