L36 MoLearn -Silurian Arisaig Series of Arisaig, N.S. 



ship with interior North America in the early Silurian. 

 Since the Beechhill fauna is not definitely correlated 

 with the American succession, its paleogeographical rela- 

 tions can not be discussed with certainty. Its affinities 

 are chiefly with Great Britain. It shows some relation 

 to the Cataract sea, hut the evidence is not conclusive. 



Ross Brook-McAdam time. — In both Ross Brook and 

 McAdam time, relationship is shown not only with Great 

 Britain, but also with eastern and western New York and 

 southward in the Cumberland trough. No relation, 

 however, is exhibited with the Mississippi sea of Indiana, 

 Ohio, etc. This is based in part on a number of forms 

 which are confined to Arisaig and northwest Europe, of 

 which Dalmanites weaveri (Salter) and the short-headed 

 Homalonotus, H. knightii Koenig, are examples. Other 

 forms are known only from Arisaig and southward in 

 New York, etc., of which Camarotcechia (Pectorhyncha) 

 obtusiplicata (Hall) and Monograptus clintonensis 

 (Hall) are examples. Others are common to northwest 

 Europe, Arisaig and southward in New York and the 

 Cumberland trough. Among these are: Plagiorhyncha 

 decemplicata (Sowerby) and Camarotcechia of the C. 

 Ihindoveriana (Davidson) type. 



Moydart-Stonehouse time. — At the end of McAdam- 

 Rochester time important paleogeographic changes took 

 place on the North American continent (1910). The 

 seas were withdrawn from the Cumberland trough and a 

 great invasion from the north spread down over the 

 interior. At this time all further communication of the 

 Arisaig sea with interior America was broken off and 

 thereafter relationship with northwest Europe only is 

 exhibited. This endured to the end of the Silurian and 

 the seas which again appeared in the Cumberland trough 

 in the late Silurian no longer display Arisaig or North 

 Atlantic affinity. Outside of a few ubiquitous species 

 there is nothing in common between the Arisaig and 

 interior seas. . This is based on brachiopod, gastropod, 

 cephalopod and trilobite evidence alone. The bivalves 

 indicate open seaways to northwest Europe but can not 

 be used as evidence of paleogeographic relation with 

 interior America of this time. For here the ecological 

 factor also enters. The prevailing clear waters and 

 hard shelly bottoms of the interior seas would' pre- 

 vent their development there anyway. Restriction of 



