1953, 1953
The Middle Ordovician of the Oslo Region, Norway. 3. Graptolites dating the Beds below the Middle Ordovician.

The lower part of the zone of Didymograptus hirundo, zone 3b ε has been found, forming the upper part of the Lower Didymograptus Shale (3b) at Slemmestad, SSW of Oslo. The development of D. hirundo and some Isograptus species is briefly discussed. A number of biserial graptolites are described from zone 3b ε. The stratigraphical importance of the occurrence of D. hirundo is mentioned.