gloss remains preserved. The roek is a real Stromatopora-limestone, 
which may be distinctly perceived in some pieces, as they consist of 
slightly curved, concentric layers the surface of which is covered 
with small knobby mamelons (25, Pl. XVII, f. 14), which make it 
more than probable that we have to do here with 
Clathrodictyon variolare Rosen sp. 
Its structure, however, is not easily traced on account of the erystal- 
line character of the stone. 
This species of boulder further contains real fossils only in the 
form of peculiar conical cavities, mostly slightly bent towards the 
point. On the inside they are invariably set with annular edges, 
which on an average are lying a little more than 1 mm. from each 
other in specimens of an average size. The cavity is often completely 
filled up with crystalline calcite bright as water. Its rather thick 
wall presents on the outside small irregularly running lines of growth. 
Frieprich Scumipt, Akademiker in St. Petersburg, whom I sent a 
piece of this limestone, was kind enough to inform me that these 
cavities originate from Cornulites sp. (1, T. 26, f. 5—8), a fossil of 
the [-zone in Oesel, frequently occurring near St. Johannis. 
These boulders are by no means rare near Groningen as appears 
from the following list: 
“Noorderbegraafplaats”’, Groningen 6 
“Boteringesingel”’, 55 ti 
“Nieuwe Kijk in ‘t Jatstraat”, 5 1 
“Nieuwe Veelading”’, a 3 
“Schietbaan’’, - Al 
Behind the “Sterrebosch’’, és 1 
6 1 
Café “the Passage’, Helpman il 
Villa “Edzes” near Haren dl 
The “Huis de Wolf” near Haren 1 
“Klein-Zwitserland” near Harendermolen _ 1 
About the occurrence of the mentioned species of Stromatopora 
NicHoLsoN records it from Borkholm and Worms in the Borkholm 
stratum in Esthonia, but he has especially found them frequently in 
the Estonus-zone there, chiefly near Kattentack. (25, p. 151). He 
does not record it from Gothland, though this fact is not sufficient 
altogether to exclude its occurrence there. Moreover LINDSTRÖM 
