W. H. Tivenhofel — Silurian Section at Arisaig. 161 



Richmond and Clinton equivalents, so well developed at Anti- 

 costi in the St. Lawrence embayment. The section practically 

 begins with the Anoplotheca hemispherica faunas, clearly of 

 Clinton time in America and the Lower Llandovery of north- 

 ern Europe. This biota maintains itself through 800 feet of 

 shales and sandstones when the continuing deposits are cut out 

 by a fault of unknown magnitude. To this division may well 

 be applied Ami's formation al name Arisaig. The guide fossils 

 of Division II are: JKonograptus clintonensis, J\£. priodon 

 chapmanensis^ Retiolites geinitzianus venosus (The equivalents 

 of these graptolites do not appear in the Norwegian section 

 earlier than the upper division of the Upper Llandovery), 

 Chonetes tenuistriatus, Anabaia anticostiana, A. depressa 

 (=Atrypa depressa Sowerby, which appears in the Norwegian 

 section at the top of the Lower Llandovery and continues 

 throughout the Upper Llandovery), Anoplotheca hemispherica 

 (restricted in the Norwegian section to the Lower Landovery), 

 Cornulites distans, and Acaste doivningiw. 



The time equivalent of these fossils is clearly Clinton, but 

 the recent unpublished work of Ulrich and Ruedemann shows 

 that this formation at Clinton, New York, not only embraces 

 the Anoplotheca hemispherica fauna, but also continues upward 

 well into the equivalent of the Rochester of western New York. 

 The Arisaig Division II is therefore thought to be equivalent 

 to the lower beds of the eastern New York Clinton, i. e., beds 

 having Anoplotheca hemispherica, and all of the Lower and 

 possibly a part of the Upper Llandovery of Norway as recently 

 described by Kiaer.* 



Division III or the McAdam formation. — Above the only 

 pronounced fault of the Arisaig section the strata continue 

 without interruption to the amgydaloidal trap. Throughout 

 a thickness of 2575 feet the faunas are continuously of one 

 progressive development, and one can trace the evolutional 

 changes of the various elements from small individuals of the 

 lower beds to the often much larger ones at the top of the 

 Arisaig section. As this series is a very thick one, it has been 

 deemed advisable to subdivide it into three divisions, and par- 

 ticularly so because this has been done by our predecessors in 

 the same field. 



Division III, consisting essentially of shales, has a thickness 

 of 1020 feet. The fauna is poorly preserved and is not a large 

 one, but when the many pelecypods are studied, a longer list 

 will be at hand than can be given at this time. Many of the 

 species extend throughout these beds to the top of Division IV. 

 As guide fossils of Division III may be mentioned Monograp- 

 tus riccartcensis, Camarotoechia neglecta, C. cf. obtusiplicata, 



* Das Obersihu* im Kristianiagebiete, 1908. 



