THE SILURIAN PERIOD. 397 



sea, there followed an expansional evolution of the shallow-water 

 fauna, and this constitutes the great biological feature of the middle 

 of the period. Toward the close of the period there was another 

 but less considerable restriction of the epicontinental sea, complicated 

 with intense salinity in the eastern interior region, and there followed 

 a second repressive evolution marked by special features. Through 

 this, the fauna passed into the Devonian type, and the marine life-history 

 of the Silurian was closed. 



These major biological movements were varied and complicated 

 by many minor movements upon which we cannot dwell. These gen- 

 eralizations relate solely to the shallow- water marine life. The history 

 of the land life should theoretically have been the reciprocal of that 

 of the sea; for as the sea contracted the land expanded, and an expan- 

 sional 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. 



It is not probable that the life of the ocean surface or of the abysmal 

 depths was sensibly changed by these continental modifications, but 

 there is practically no evidence bearing on this point, for, as ever to 

 be borne in mind, the geological record of marine life is almost wholly 

 that of the shore tracts and the epicontinental seas. 



Had the physical changes of the continent at the close of the Silurian 

 period been as pronounced as those at its beginning, it would have 

 constituted an ideal cycle, consisting of an advance, a dominance, 

 and a retreat of the sea, attended by expansional, cosmopolitan, and 

 restrictional phases of shallow-water life, and would thus have rounded 

 out a typical period. Starting with a transformed remnant of the 

 previous fauna distributed along a skirting shoal tract and gathered 

 in a few bordering embayments which harbored it during the vicissi- 

 tudes of the transition, the life expanded with the expanding epicon- 

 tinental seas, attained a cosmopolitan prevalence in the Mid-Silurian, 

 and later, with the retraction of the sea, shrank again to meager pro- 

 portions and was transformed into the initiative fauna of the next 

 period. 



The Transition from the Ordovician. 



Severely restricted as the shallow-water life must have been during 

 the stage of the greatest sea withdrawal, the record probably makes 



