THE SILURIAN PERIOD. 399 



It will claim consideration in connection with the great red deposits 

 of later periods. 



Universally adverse conditions are not, however, to be assumed 

 during the transition epoch. The very drawing down of the waters 

 on the borders of the continent must inevitably have so shallowed 

 the sea on some deeply submerged tracts as to have made them con- 

 genial ground for the retreating faunas of the interior. This might 

 well be true of deep embayments on the continental border. The 

 Gulf of St. Lawrence seems to have been a notable example, and to 

 have constituted a harbor of refuge for the retiring hosts of the interior. 

 In it, as shown by the deposits of Anticosti, 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 at all probable that it was the only one. Doubt- 

 less there were other similar embayments, and the whole coast tract 

 probably made its contribution to the Silurian fauna; but south of the 

 St. Lawrence the record of the phases of life transitional from Ordo- 

 vician to Silurian is not at command. The main Silurian fauna of 

 the interior apparently did not arise from that of the St. Lawrence 

 embayment or of the Atlantic border, but developed somewhere at 

 the north, or came in from the eastern continent. 



The Expansional Stage and the Mid-Silurian Fauna, 



As the sea slowly crept out upon the face of the continent toward 

 the middle of the period, increasing room and more congenial condi- 

 tions for most forms of shallow-water life afforded the means of a new 

 expansional evolution, and this finds expression in the Niagaran fauna. 

 This is comparable to the expansional stage of the Mid-Ordovician 

 fauna. The families and classes remained nearly the same as in Ordo- 

 vician time, but the majority of the genera were new, as were nearly 

 all the species. A very few species of extraordinary longevity bridge 

 the great interval, such as Leptana rhomboidalis (Fig. 163, w), Plectam- 

 bonites sericeus (Fig. 163, x), and Platystrophia biforata. These stand 

 out as conspicuous examples of extraordinary persistence, though 

 even they had changed somewhat. In general there was a biological 

 advance, but regarding the particular classes, some had risen and 



