THE SILURIAN PERIOD 547 



epicontinental sea, complicated with intense salinity in the eastern 

 interior region, and there followed a second repressive evolution by 

 which the fauna passed into the Devonian type. 



Theoretically, the history of the land life should have been the 

 reciprocal of that of the sea; for as the sea contracted, the land 

 expanded, and an expansional evolution of land life should have 

 run hand in hand with the restrictional evolution of the sea life. 

 This was probably the fact, but the record of the land life is too 

 meager to , demonstrate it. In so far as the climate was arid, it 

 was unfavorable for abundant land life. 



The Transition from the Ordovician 



Of the shallow-water life of the early Silurian there is but meager 

 record. The eastern shore of the continent was then far out on the 

 borders of the continental platform, and the deposits there are 

 buried and inaccessible. The western border may have been sub- 

 merged, but the fauna there is little known. 



Aside from the lessened area favorable for shallow-water life, 

 the conditions were probably less favorable, area for area, than be- 

 fore, for the wash from the land was presumably increased. The 

 increased detritus brought to the sea probably inhibited some 

 forms of life, injured others, and helped but a few. Some of the 

 basins and bays were doubtless too fresh and some too salt, and 

 some may have varied unfavorably in salinity. These general 

 considerations may explain the meagerness of the faunas of the 

 early Silurian strata. But conditions were not adverse every- 

 where. In the Gulf of St. Lawrence, Ordovician species lived on 

 for varying lengths of time, and mingled with Silurian species as 

 they developed, and so recorded the transition. This appears to 

 have been the breeding-ground of one of the provincial phases of the 

 Silurian fauna, but it is not probable that it was the only one. The 

 main Silurian fauna of the interior apparently did not spring from 

 that of the Atlantic border, but developed somewhere a,t the north, 

 or migrated from Europe by a northerly route. 



The Expansional Stage and the Mid-Silurian Fauna 

 As the sea slowly overspread the continent toward the middle 

 of the period, increasing room and more congenial conditions for 



