562 NEW YORK STATE MUSEUM 



Dichograptus sensu stricto is greatly predominant, would indi- 

 cate that there may exist in the Main Point Levis zone a still 5 

 older subzone, carrying principally these species of Clonograp- 

 tus, and which is not exposed in the Deep kill section. The 

 latter snbzone would then correspond to the lower Tetragraptus 

 beds, and this Deep hill subzone to the Dichograptus beds. This 

 correlation is supported by the fact, that the next Deep kill 

 zone is homotaxial with the upper Tetragraptus beds. 



The investigations of Hopkinson and Lapworth 1 have demon- 

 strated that the characteristic fossils of this zone occur also in 

 the Arenig series of St Davids in Wales, of Shelve in West 

 England, and in the Ballantrae terrane, underlying the Moffat 

 series in south Scotland. 



The most detailed division of the graptolitiferous beds and 

 the most exact correlation with the limestone facies have been 

 attained in southern Sweden where the paleozoic beds, as a rule, 

 lie horizontal. It is only necessary to cite the names of Lin- 

 narsson, Tornquist and Tullberg to indicate the refined division 

 in zones of the Siluric in that country, which now has become 

 common property by its adoption in textbooks. 



Linnarsson 2 comprised under the name lower Graptolite schists 

 (or Phyllograptus schists, as proposed by Dr Tornquist) all the 

 graptolite-bearing strata that lie between the Ceratopyge and 

 Orthoceras limestones. Their fauna consists of the Dichograp- 

 tidae and their closest ally the Phyllograptidae. He points out 

 that the most abundant species are identical with or representa- 

 tive of those familiar to us in the Skiddaw and Quebec groups, 

 such as Didymograptus patulus, constrictus,. 

 indentus, etc., Tetragraptus quadribrachi- 

 atus, bryonoides and b i g s b y i . There can be, hence, 

 no doubt that this zone is identical with the lowest Deep Mil zone, 

 called the Tetragraptus zone in this publication. 



1 See specially Ann. and mag. nat. hist. 1879. 5th ser. 2: 4, and GeoL 

 mag. 1889. 3d ser. 6: 20, 59. 

 2 Geol. Foren. Forhandl. Stockholm. 1879. no. 8, p. 227 ff. 



