144 THE HABITAT OF THE EURYPTERID A 



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 Sphasrocodium 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 Rootzikiiil 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, 11 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. Simonsonhe 

 obtained a slab which showed the portion of the abdomen and cara- 

 pace 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 remain- 

 ing 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 

 Rootzikiiil the eurypterid-bearing plattenkalk appears at the surface 



11 This species was not collected by Schmidt but was described by him from a specimen from 

 Volborth's collections. 



