Mr Learn — Silurtaii Arisaig Series of Arisaig, N. S. 133 



study of the salinity, temperature and depth factors indi- 

 cates that they cannot even be considered as contribu- 

 tory. It is well to note in passing that all shales do not 

 record the same environmental conditions. Thus many 

 calcareous shales must have accumulated quite slowly, 

 for at times it was possible for thin layers of pure lime- 

 stone to form within them. Such shales have faunal 

 associations more closely related to those of pure lime- 

 stone. Biological associations similar to those of Ari- 

 saig are found in the late Silurian (Upper Ludlow) of 

 Great Britain, where similar groups of bivalves are 

 related to similar mud bottoms, i. e., to those of marginal 

 but normally marine seas receiving the silt-laden waters 

 of inflowing rivers from a high landmass. The adapta- 

 tion may not be to mud alone, but also to food supply 

 associated with the incoming silt. 



So the Arisaig fauna becomes an index of response 

 to such a marine environment and the faunal associations 

 described on a preceding page record the relative adap- 

 tability of various groups of organisms to it. The 

 degree of adaptation varies considerably, some forms 

 being restricted to one or another environment while 

 others are more "indifferent." With the exception of 

 a few species scattered through various phyla, it is only 

 within the bivalve families Grammy siidse, Nuculacea and 

 Modiolopsidse that a preference is prevailingly shown 

 for the mud-bottom environment. Within them many of 

 the genera and a preponderance of the species of the 

 remaining genera are restricted to the shale beds. These 

 then are primarily the mud-dwellers of the Silurian seas. 

 The representation in this environment of all other fami- 

 lies and phyla seems to depend on the resistance of some 

 hardy forms to these, for them, unfavorable conditions. 

 These include the "indifferent" organisms which have 

 a wide environmental range and also the few species 

 actually adapted and restricted to this mud habitat. 



Correlation. • 



Owing to the peculiar biological associations of this 

 fauna, it has fewer species than usual in common with 

 other faunas. This makes the correlation difficult. 



Great Britain. — The best comparisons can be made 

 with the British Silurian. These are very important, for 



