132 McLeam Silurian Arisaig Series of Arisaig, W. S. 



groups of organisms, since the variety of life declines in 

 the cold waters. But this factor would not bring about 

 the profound modification of this fauna and certainly 

 would not result in a diversity of bivalves. Moreover, 

 the climates of the Silurian seem to have been uniformly 

 distributed. They were warm rather than cold, and in 

 the earlier Silurian, reef corals flourished not far away 

 in the Anticosti sea. No ecological factor related to 

 depth could be called upon to explain this faunal associa- 

 tion and the geological test of constancy throughout a 

 whole period could not be met. 



Among the known and larger factors, this leaves, by 

 elimination, the control of the mud bottoms and turbid 

 waters. The proof of their potency, however, need not 

 be left to rest on negative evidence alone, for there is 

 much of a positive kind in their' favor. Criteria of a 

 positive nature may be sought in the local distribution 

 of the fauna. This may be called the internal evidence. 

 It is found that the bivalves which are, in particular, 

 characteristic of this locality, are confined to the shale 

 beds, i. e., they were mud-dwellers and avoided the harder 

 shelly bottoms. It is not a far step, therefore, to seek in 

 the prevalence of mud bottoms the cause of this rich 

 bivalve development. That the depauperation in other 

 groups is related to the muddy conditions is suggested 

 by their better, although still restricted, development in 

 the impure limestone lenses as compared with shale. 

 This can be well tested in the Stonehouse, the fauna of 

 which embraces two distinct animal communities, one a 

 mud faunule chiefly of bivalves, and the other a shell- 

 bottom faunule, with few bivalves and richer in all other 

 forms of life. 



We can also examine the external evidence. It may 

 be recalled that there is no true limestone in the Arisaig 

 series. It is found that the forms of Silurian life absent 

 from Arisaig are elsewhere most common in limestone 

 or calcareous shale with pure limestone layers. This 

 means that the absent forms are inhabitants of clear 

 waters and suggests that it was the muddy bottoms and 

 silt-laden waters of the Arisaig sea that prevented their, 

 entrance there. 



Therefore, both the internal and external evidence sup- 

 port the conclusion that it was the silt factor that con- 

 trolled the biological associations of this fauna, while a 



