( 507 ) 
and thus forms a real shell-breccia. The ground-mass then is commonly 
weathered to a more or less earthy yellow mass, which also covers 
the surface of the shells, by which the whole assumes a yellow 
colour. In other cases, however, the weathered ground-mass is almost 
white, sometimes also brown-ochre-yellow. Though they differ so 
much in exterior appearance, all specimens have in common that 
this Pentamerus occurs almost always only in single valves which 
themselves are, for the greater part broken into more fragments. 
I have never been able to produce a wholly preserved specimen, 
though some fragments actually show that parts of both valves occur 
in natural position. So this confirms in the main the results of 
Rormer’s examination (13, p. 74), though I doubt of the truth of 
his opinion, according to which these boulders should contain only 
ventral valves of this species. This conception was supported by 
KIcHWALD’s communication that also in the parent rocks both valves 
were never seen in connection. Nevertheless Ererwarp did know the 
smaller dorsal valve and describes it as having half the length of 
the larger one, being much broader and much less vaulted. This 
can hardly be right, for afterwards Scumipr found complete specimens 
at Weissenfeld in the neighbourhood of Hapsal in Esthonia. Among 
my material for comparison there are three such specimens from 
the above-mentioned place, collected by Boxnema. These, however, 
show a dorsal valve, but little smaller than the ventral one, but 
much flatter and so comparatively wider. This causes the great 
difference between the two shells to disappear, and so there is no 
reason left for the inexplicable fact that in boulders only the ventral 
valve should occur. Meanwhile the interior structure of the small 
shell has to be examined still to confirm this. I have not been 
occupied with this work. 
6. Dolomite, very fine-grained, sometimes even impalpable, of a 
light-gray or light-brownish-yellow colour. This dolomitie ground- 
mass also contains great numbers of nuclei of Pentamerus borealis 
Ercuw., which are covered all over with little, graceful, dolomite- 
rhombohedra, which, however, are easily perceptible by the naked eye. 
Of these boulders, which in literature is usually called “Penta- 
merenkalk” are found here : 
Limestone : Boteringesingel, Groningen 2 
Behind the “Sterrebosch’, _ ,, A 
Helpman 1 
the “Huis de Wolf”, near Haren 1 
“Old Collection” 9 
Dolomite : Boteringesingel, Groningen 2 
