Page:The Habitat of the Eurypterida.djvu/153

Rh  sp., Laceripora cribrosa, but no eurypterids. In the quarry at Ladjal itself, in a band of limestone apparently in place, there occurred a great mass of Leperditia baltica, and also Spirigerina didyma, while in marly interbedded layers Eurypterus fischeri occurred in traces. To the southeast this limestone merges into solid gray limestones carrying trilobites, crinoids, brachiopods, etc., but not eurypterids. At Nessoma, southeast of Sandel occurs an outcrop of the upper crystalline limestones which marks the Spirigerina prunum horizon, and in intercalated brown marly layers were found great numbers of fish scales and breast plates, similar to those occurring at Ohhessare-Pank on the southwestern end of the island. The section at Lode about the same distance west of Arensburg as Nessoma is east of it, has brought to light one of the richest collecting grounds on the island for the typical marine forms. Here the rock is a gray limestone in which Spirigerina prunum occurs in great numbers but is not well preserved; Leperditia baltica is occasionally found, but the abundant forms are: Calymene blumenbachii, Orthoceras bullatum, Spirifer elevatus, Orthis orbicularis and Chonetes striatella, all characteristic of the Upper Ludlow of England (Schmidt, 241, 176–7).

In summary, it may be said that the detailed sections bring out the sporadic occurrence of the eurypterids in very thin beds, rarely intimately associated with the typical marine forms which occur in beds above and below the eurypterid marls. As the beds are traced to the south, southwest and southeast they are seen to be replaced by those containing a pure and abundant marine fauna, but not a trace of a eurypterid. Moreover, it is apparent that the occurrences are in all cases immediately associated with the physical and faunal evidences of a break in the series between beds of Lower and Upper Ludlow age, and that this is essentially the horizon at which the eurypterids and Palæophonus nuncius are found on the island of Gotland, marking in both cases what seem to be widespread river deposits which precede the renewed encroachment of the sea in Upper Ludlow time.