McLearn — Silurian Arisaig Series of Arisaig, N. S. 131 



familar groups of Silurian life. The almost entire 

 absence of corals claims first attention, while the lack 

 of Stromatoporidae and the impoverishment in Crinoidea 

 and Bryozoa do not escape notice. The Brachiopoda, 

 outside of some common species of Dalmanella, Chonetes 

 and Camarotcechia and a few other genera, are poorly 

 represented. Many ubiquitous species and with few 

 exceptions all the higher and more specialized families, 

 in particular the Pentameridas and the spire-bearers, are 

 missing. As if to atone for this, the bivalve families 

 Grammysiidae, Nuculacea and Modiolopsidae have a rich 

 development and make a strong impress on the total 

 assemblage of the fauna. The Aviculidae have about the 

 normal development, but the bivalve Cardiolidae, Cono- 

 cardiidae and Megalomidae are conspicuously absent. 

 The remaining phyla, the Gastropoda, Pteropoda, Cepha- 

 lopoda, Trilobita and Ostracoda, make a very modest 

 showing. Compared with the majority of Silurian 

 faunas, the differences lie chiefly in a falling off in many 

 groups and the only new and unfamiliar elements intro- 

 duced are to be found among certain families of bivalves. 

 While it is natural to turn to the prevailing muddy 

 bottoms as the cause of this faunal association, it must 

 be remembered that there are other factors of marine 

 environment. These must be considered and the evi- 

 dence for the control of the mud factor must be examined. 

 Curiously enough, some of these other factors tend to pro- 

 duce somewhat similar biological associations. A reduc- 

 tion in salinity would early be marked by an elimination 

 of corals and then by a falling off within the other phyla, 

 including the bivalves. Finally, it would result in a 

 bivalve fauna largely, with some gastropods and crusta- 

 ceans, including contributions from the fresh-water 

 realm. But the molluscs would be poor in variety of 

 development and in species. It would by no means 

 explain the rich bivalve development of this fauna, for 

 such diversification is only possible in the normal marine 

 realm. Moreover, water bodies of decreased salinity are 

 evanescent in geological time, as recently pointed out 

 by Miss O'Connell (1916), whereas the factor or factors 

 that controlled the biological associations of this fauna 

 endured without relaxation throughout nearly a whole 

 period. Reduced temperature also would finally result 

 in the elimination of corals and in a depreciation in all 



