ART. 21 ORDOVICIAN TBILOBITES ULEICH 65 



southern genera appeared in the temperate zone of the northern 

 hemisphere — was noted in looking over Twenhofel's recently pub- 

 lished work on the Geology of Anticosti Island. In this report 

 Twenhofel illustrates and very briefly describes four Lichadidse, 

 three of them new and all referred to the genus Amphilichas. The 

 oldest of these, A. horealis, new species, founded on a cranidium col- 

 lected from the upper half of the English Head formation, is a 

 normal Amphilichas and much closer to other American Richmond 

 species than to the Swedish A. dcdecarlicus wnth which Twenhofel 

 compares it. However, the other three Anticosti species belong to 

 two of the genera above listed as Silurian migrants from the south. 

 One, A. shallopensis Twenhofel, from zone 9 of the Jupiter River 

 formation, is a typical Lichas; the other two — A. canadensis (Bill- 

 ings), from the lower half of the Jupiter River, and A. arenaceus 

 Twenhofel, from the top bed of the underlying Gun River forma- 

 tion — are based on such characteristic pygidia that I refer them 

 without hesitition to Arctinunis. The presence of species of Lichas 

 s.s. and Arctinwncs in Anticosti and their restriction here to zones 

 that all agree are of the age of the Clinton are facts that for three 

 reasons are regarded as of particular significance in the determination 

 of the age relations of the Keisley and related north European de- 

 posits to formations of the American Paleozioc sequence. First, 

 .because unquestioned Richmond formations (English Head and 

 Vaureal) are succeeded in the Anticosti section by two formations 

 (Ellis Bay and Becsie) that I regard as representing the Upper 

 Medina or Alexandria group of New York and the Mississippi 

 Valley and which in turn are succeeded by the Gun River and Jupiter 

 River formations, which contain the mentioned species of Lichas and 

 Arctinurus and of which the latter and at least the upper part of the 

 former are undeniably of Lower Clinton age. The second reason is 

 the well-known fact that the faunas of the Clinton part of the Anti- 

 costi section exhibit closer relations to British Llandovery and Wen- 

 lock faunas than any other Silurian section in North America. The 

 third reason is that in Europe as in America the above listed genera 

 of brachiopods and trilobites that have been set down by British and 

 most European geologists as common to Ordovician and Silurian 

 deposits in their countries attain their best development in Upper 

 Clinton and later Niagaran deposits ; and they do not occur at all in 

 America beneath the top of the Medina nor in Britain and Sweden 

 beneath the formations that I claim would be more naturp.lly classified 

 as early Silurian than late Ordovician. 



The misunderstandings that have so long beclouded the problem 

 of the proper position of the Ordovician-Silurian boundar}^ have 

 64441—29 5 



