( 148 ) 
in the midst a wedge-shaped elevation. The cheeks are in front 
separated by a furrow running from the front of the glabella to 
the edge-furrow. 
The tail-shields are much more vaulted. This is especially the 
case with the rhachis, which broadens towards the back and stretches 
nearly as far as the edge. Consequently, the lateral parts of the 
pygidium, which are already narrow, become even more so towards 
the back part. They are not separated by a furrow, as it is the case 
with those of the head-shields. The pygidia have at the back-edge 
on either side a little cog pointing backward. The rhachis of the 
pygidia is clearly divided into three parts. The back-part is the 
largest by far and is particularly swollen. The lateral farrows of 
one side do not meet those of the other, as they are separated by a 
wedge-shaped elevation passing on from the second part to the first 
and ending towards the back in a blunt point slanting upward. 
From the properties mentioned it may be easily seen why this 
kind of Agnostus was described by TULLBERG*) as Agnostus pisifor- 
mis L. var. socialis. Pictures of it have been given by BRÖGGER ®) 
and PoMmPrckr *). 
Up to this time this erratic-block is the only piece of alum-slate 
with Agnostus pisiformis L. var. socialis that was found in our 
diluvial grounds. In Germany they also seem to be very rare. Only 
GorrscHe *) mentions such a piece from Scnvrav. This one also 
contains, however, remains of Olenus truneatus Briinn. As firm rock 
such alum-slate with this variety of trilobites occurs in Sweden 
(Oeland and Bornholm included), in different places, as I learned 
from Prof. Mopere, to whom I showed a piece of the erratic-block. 
Microbiology. — “Accumulation experiments with denitrifying 
bacteria’. By G. VAN Irerson Jr. (Communicated by Prof. 
M. W. Beijerinck). 
The great signification of the denitrifying bacteria for the circulation 
of nitrogen in organic life and the important biochemisms to which 
they give rise, made the study of these organisms very attractive. 
1) Tuttperc, Om Agnostus-arterna 1 de Kambriska aflagringarne vid Andrarum. 
Sveriges geologiska Undersökning. Ser. CG. N° 42 pag. 25. 
2) Bréecer, Die Silurischen Etagen 2 und 3 im Kristianiagebiet und auf Eker. 
Pag. 56. Taf. 1. fig. 10abe. 
3) Pompeckt, Die Trilobiten-Fauna der Ost-und Westpreussischen Diluvialgeschiebe. 
Beiträge zur Naturkunde Preussens herausgegeben von der Physikalisch-Oekono- 
mischen Gesellschaft zu Königsberg. Pag. 15, Taf. IV, fig. 24a b. 
4) Gorrscue, Die Sedimentär-Geschiebe der Provinz Schleswig-Holstein, pag. 11, 
