the Ilionia beds in Gotland, Murchisonia compressa (Gotland VI), and Spirigera (= Meristina) didyma, which is the most widespread form in the northern outcrops of zone K in Oesel and which occurs at Visby in the top of bed III, in the Sphærocodium marl below the Ilionia limestone, and above the eurypterid marl of Gotland, as well as in the Aymestry limestone and Dayia beds of England, all (except possibly the last two) of Upper Ludlow age. Thus there is evidence of a faunal break in the series, since beds containing Upper Ludlow fossils everywhere in eastwest sections across central Oesel follow upon beds with Wenlock fossils. In many localities the indications of a physical break are also present as may be best shown in a few detailed sections.
The fullest development of the eurypterid fauna is seen in the rocks underlying Rootziküll on the west coast of the island of Oesel in the parish of Kielkond. Here the beds of the lower part of zone K are a fine-grained "plattenkalk" or dolomitic calcilutyte, in which the chitinous exoskeletons of Eurypterus fischeri Eichwald, E. laticeps Schmidt and Pterygotus osiliensis Schmidt have been so excellently preserved. Associated with the eurypterids in the same bed have been found the tail of Ceratiocaris notlingi Schmidt,[1] the shields of two cephalaspid fishes Thyestes verrucosus Eichw. and Tremataspis schrenkii Schmidt, and the shells of the little Lingula nana Eichwald. Nearly fifty years after these first discoveries Schmidt was able to add a new species to the fauna perhaps representing a genus not heretofore known outside of North America. From A. Simonson he obtained a slab which showed the portion of the abdomen and carapace of this new species which he called Stylonurus (?) simonsoni (252, 157).
Attention has already been called to the fact that the eurypterid exoskeletons have the original chitin still preserved and that this may be lifted from the rock so that both the upper and under surface and the sculpture thereon may be studied. The shells of the remaining fossils which are found in this bed are destroyed; these include the rarely occurring Hemiaspids: Bunodes lunula Eichw., B. rugosus Nieszk. and B. schrenckii Nieszk sp. as well as Pseudoniscus aculeatus Nieszk. and the shells of Orthoceras tenue Eichw. All of these forms are represented only by carbonaceous films. In the environs of Rootziküll the eurypterid-bearing plattenkalk appears at the surface
- ↑ This species was not collected by Schmidt but was described by him from a specimen from Volborth's collections.
