232 bulletin: museum of comparative zoology. 



Lower Middle Ordovician. 



As has already been stated, there is throughout northern Europe, 

 a distinct change in fauna after the deposition of the last of the strata 

 correlated above with the Beekmantown of America. 



In Russia, in Vastergotland, and on Oeland, where there is a con- 

 tinuous section of calcareous strata this change is less marked than in 

 Norv.-ay, where the Gigas limestone is succeeded by black shales, but 

 it is shown as well in Scania where the strata of the section are mostly 

 shales. 



In Russia, Ci marks the introduction of the Echinosphaerites- 

 Christiania fauna, in which nearly all the species are different from 

 those found in the strata below. In Norway, the Gigas limestone is 

 followed by black shales with the Didymograpfus gcminus fauna, and 

 in Scania the shale with PhyUograptus cf. iypus is followed by shale 

 with Didymograptus geminus. On Oeland, the Gigas limestone is 

 followed by limestone with Asaphus platyurus, and that in turn by 

 limestone with Illaenus chiron and Didymograptus geminus. Through- 

 out Scandinavia then, the zone with Didymograptus geminus is the 

 earliest zone of the series succeeding the Arenig. In Great Britain 

 likewise there is a zone of Didymograptus murchisoni, a species practi- 

 cally identical with D. geminus, which is considered as the lowest zone 

 of the Llandeilo. In Great Britain there are two species of Dicello- 

 graptus in the fauna of this zone, and no species of either PhyUo- 

 graptus or Tetragraptus are present. The latter statement is true 

 also of Scandinavia, and the zone is known at all localities as being 

 the oldest in which there is a profusion of diplograptids. This zone 

 is not yet positively identified in America. Ruedemann has pro\ i- 

 sionally correlated the Bed 7 of the Deep Kill section of New York 

 with the European strata containing the Didymograptus murchisoni 

 fauna, but although PhyUograptus and Tetragraptus were not found 

 by Ruedemann, the remainder of the faima is so nearly identical with 

 the fauna of the Diplograptus dentatus zone of the Levis section, where 

 both Tetragraptus and PhyUograptus do occur, that I am inclined to 

 believe that the zone 7 belongs with the Levis. 



In Scandinavia the Didymograptus geminus beds are followed by the 

 Lower Dicellograptus shales, containing the zones, in ascending order, 

 of Glossograptus hincksi, Climacograptus putillus, and Nemagraptus 

 gracilis. Unfortunately, a great deal remains to be done before a very 

 satisfactory correlation of the Echinosphaerites limestone of Noi-way 



