Geology. — “Contributions to the knowledge of the sedimentary 
boulders in the Netherlands. 1. The Hondsrug in the province 
of Groningen. 2. Upper Silurian boulders. Second communi- 
cation: Boulders of the age of the Eastern Baltic zones H and 
I’ By Dr. H. G. Jonker. (Communicated by Prof. K. Martin). 
Hi. 
Besides the Borealis-limestone, described in my preceding commn- 
nication (33) and on which I am going to touch later on, boulders 
with Pentamerus-remains near Groningen are rare. I can mention 
but three pieces here, in two of which the species is not to be made 
out, while in the third, found in the “Noorderbegraafplaats” in Gro- 
ningen, Pentamerus estonus Eicuw. occurs. Nor is this determination 
beyond doubt and especially the possibility of its being Pentamerus 
oblongus Sow. can in my opinion not be excluded, as indeed in out- 
ward appearance the latter corresponds almost perfectly with the 
former (12, p. 81 and 3, T. XVIII, f. 4°). As however, the latter 
form in Gothland has no doubt to be looked upon partly as the 
real P. estonus EKicuw. (27, p. 98), nothing can be said for certain 
about its origin, as the rock, a weathered, yellow limestone does 
not give sufficient indications for it. | mention this boulder however 
for completeness’ sake. 
With regard to the Borealis-limestone I wish to add, that after 
all I did find an almost complete specimen of Pentamerus borealis 
Eicuw., in the Groningen museum, evidently from a Groningen 
boulder. The correspondence with the specimens from Weissenfeld, 
mentioned before, is however not very great, the top of the ventral 
valve in our specimen being much more curved and thus agreeing 
more with E1cuwanp’s deseription. 
A close investigation removing the existing confusion with regard 
to the Upper Silurian Pentamerus-species is really most desirable. 
31. Clathrodictyon-limestone. 
White limestone, sometimes having a more or less light-yellowish- 
gray tinge. At the surface and in cavities the colour is rather yellow. 
It is always crystalline and the very irregular fractured surfaces 
show a peculiar fatty silk-gloss, which is most characteristic of them. 
If the colour becomes a little darker, as is sometimes the case, the 
