J. HALL — THE GENUS SPIRIFERA. 567 



lakes ; but on the eastern half of the continent the principal drainage system, carrying 

 its vast freight of detritus west to the Cretaceous ocean, is probably marked by the 

 chain of great lakes from Ontario to Superior, the west end of which is close to the 

 east border of the Cretaceous belt. At that time and afterward much of this eastern 

 land area was elevated at least several hundred feet above its present level, so that 

 streams flowing where these great lakes now are, eroded their basins, then lying 

 wholly above the sea level and sloping westward. It seems possible also that other 

 great tributaries may have flowed west and south into this Cretaceous sea, bringing 

 sediments eroded from the areas of Hudson's bay, Lake Athabasca, and Great Slave 

 and Great Bear lakes. Amid the subsequent changes of level which have perma- 

 nently uplifted the Cretaceous sea-bottom in the center of the continent, and have 

 uplifted and afterward depressed our northern coasts, both on the Atlantic and the 

 Pacific, the writer. believes that the basins of the Laurentian lakes, while still contin- 

 uous areas of valley erosion, were raised with the country east and west to a great 

 altitude for a short time at the end of the Pliocene period, as shown by deep stream- 

 courses enveloped by the drift deposits, but that in the Quaternary depression, by dif- 

 ferential subsidence, these basins became divided from each other, their bottoms, 

 excepting that of Lake Erie, sinking beneath the level of the sea. The avenue of 

 outflow from them has been turned to the northeast, forming the Eiver St. Lawrence, 

 in the Glacial period. President Chamberlin believes that much subsidence of the 

 beds of these lakes probably is attributable to the weight of the ice-sheet. The post- 

 glacial re-elevation, which has produced the northward ascent of the beaches of the 

 glacial Lake Agassiz and of the contemporaneous higher stages of the Laurentian 

 lakes, has failed to raise these lake beds, as likewise the bottoms of the fiords, to the 

 present sea level. 



The next paper was presented in abstract only, and follows in brief 

 synopsis : 



ON THE GENUS SPIRIFERA, AND ITS INTERRELATIONS WITH THE GENERA 

 SPIRIFERINA, SYRIXGOTHYRIS, CYRTIA AND CYRTINA. 



BY JAMES HALL. 



[Synopsis.] 



1. Great development of Spiriferg, in American Paleozoic. 



2. Previous classification of species. 



3. External ornamentation as a basis of classification. 



NORMAL FORMS. 



(.4) radiata 

 (B) Ifime/losa 



(C)fimbriafa ] j- Begin in the Niagara. 



fimbriaia-plicata j- reticularia. McCoy. 



" undid ata j J 



ABERRANT FORMS. 



(D) Icevls. Begins in the Corniferous. 

 Ambocoelia differs internally. 



