222 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
SUMMARY. 
From the above summary of the sections in the principal Ordovician 
belts of Sweden, certain facts should appear. 
It will be noted that the Dictyonema zone is present in almost all 
sections, and that the character of the deposit is controlled by the 
underlying beds, a fact which in itself is evidence of a cessation of 
sedimentation, uplift, and erosion. In North Oeland, where the 
basement beds are the Paradowides tessini sandstone, an Obolus sand- 
stone is developed, and in the northern belt, Dalecarlia-Gastrikland, 
where the older strata are pre-Cambrian crystallines and Lower 
Cambrian sandstones, one finds Obolus conglomerate and Obolus 
sandstone. That the Obolus sandstone is of the same age as the 
Dictyonema shale is of course abundantly proved by their interstratifi- 
cation with each other at several places in Esthonia. In southern 
Oeland, in Scania, and at Christiania, where the underlying strata 
are the black shales and limestones of the Upper Cambrian, the Dictyo- 
nema zone is developed as a shale without sand and has even, in places, 
irregular layers of limestone. 
Over a certain area, in Nerike and part of Vastergétland, the 
Dictyonema zone is absent. Thus, at Kinnekulle the Ceratopyge 
limestone rests upon the limestone and shale of the Upper Cambrian, 
at Ekedalen the Planilimbata limestone rests directly on the Upper 
Cambrian, and at Hunneberg the Ceratopyge limestone rests on the 
Upper Cambrian limestone in most places, though in some spots about 
three inches of shale belonging to the Dictyonema zone have been 
reported. The arrangement of these localities free from Dictyonema 
deposits suggest a large, low lying island composed of uplifted Upper 
Cambrian strata, which was progressively submerged during Lower 
Ordovician time, but not completely covered till toward the end of 
Planilimbata time. Correlated with this may be the distribution of 
the Lower Didymograptus shale. 
As has already been pointed out repeatedly by the Swedish geolo- 
gists, when the Lower Didymograptus shales are present in any sec- 
tion, the Planilimbata limestone is absent, and when the latter is 
present, the former is absent. This has naturally led to the deduction 
that the two formations were deposited at the same time and owe their 
dissimilar faunas to the very different conditions of sedimentation. 
On this point, the evidence, though strong, does not seem to be abso- 
lutely conclusive. It would seem that, should it happen that lime- 
