Most recent investigators have placed the Silurian-Devonian boundary at the base of the Monograptus uniformis Zone. In the Arctic there are several graptolite-bearing clastic and clastic-carbonate sequences where this boundary may be drawn, but correlation of these separate sections with more widely distributed carbonate formations is tentative. The boundary position within carbonate sequences is unclear.

The best key marker traceable throughout the Arctic consists of argillaceous limestone containing Atrypella phoca and A. scheii. This marker bed near the Silurian-Devonian boundary is important because it is easily recognizable elsewhere.

In the western Soviet Arctic, deposits containing A. phoca and A. scheii are assigned to the Greben Horizon. At its type section on Vaygach Island, the Greben Horizon lies on carbonate rocks of the Khatanzeya Horizon (Ludlow). The gray and greenish-gray weathered nodular limestone of the Greben, about 200 m thick, is characterized by a rich fauna which includes A. phoca and A. scheii. On the basis of the fauna, the Greben Horizon has been dated as Silurian--either Ludlovian or post-Ludlovian.

In the overlying Vaygach Horizon, bioclastic detrital limestone with bioherms grades upward into predominantly algal limestone. The Vaygach Horizon, which is characterized by the presence of tabulates, may correspond to the post-Ludlovian-pre-Gedinnian and Gedinnian deposits.

In the western Soviet Arctic, the genus Atrypella has wide vertical distribution, but A. phoca and A. scheii are characteristic only of the Greben Horizon, especially of its lower part.

Atrypella phoca and A. scheii are the most characteristic faunas of the Read Bay Formation (late Wenlockian to late Ludlovian or Gedinnian) and its equivalents in the Canadian Arctic Archipelago. The Read Bay Formation at its type section on Cornwallis Island corresponds to the whole Khatanzeya and Greben Horizons.

The Sutherland River Formation on Devon Island is of Late Silurian or Devonian age; it has been correlated with the Snowblind Formation, which lies on the Read Bay Formation. The Sutherland River fauna includes forms similar to those of the upper Greben and lower Vaygach Horizons. The Greben Horizon appears to correlate with the part of the Upper Silurian in southeastern Alaska which contains Atrypella scheii and Camarotoechia sp.

The Lower Devonian Salmontrout Limestone of the Porcupine basin corresponds to the rocks of the Pragian and Zlikhovian in the Soviet Arctic.

Correlation of Lower Devonian deposits of the Soviet Arctic with deposits of the Canadian Arctic Islands is complicated and tentative. The Blue Fiord Formation, considered by the writer to be upper Emsian, contains fossils typical of the upper Valnev Horizon and the lower Favosites regularissimus local zone in Novaya Zemlya. The Stuart Bay Formation on Bathurst Island may correspond to the Valnev Horizon. The overlying Eids Formation contains fossils present elsewhere in both upper Emsian and Eifelian rocks.

The deposits underlying the Gedinnian and those equivalent to the upper Emsian are the most promising levels for correlation of Lower Devonian deposits in the Arctic.