462 A. W. GEABAU THE SIIERBURXE SAXDSTOXE 



pared with 60 Hamilton species. Of the remaining 10 forms. 3 are hold- 

 overs from the Traverse fauna of the interior, and T are immigrants 

 from the Xaples fauna. 



In the next table (Y, page 4:61) the species from the eastern sections 

 are analyzed from Prosser's determinations. 



An inspection of this table will show that out of a total of 124 specific- 

 ally identified forms. 9T, or T8.23 per cent, including 22 doubtful forms, 

 are Hamilton t}3)es. while 15. or 12.1 -per cent, are immigrants from the 

 Ithaca region, where the indigenous Portage fauna developed — that is, 

 west of the Sheburne bar. 



Of the 12 remaining species, 3 are immigrants from the Xaples fauna, 

 two are urmamed varieties of Hamilton species, and 7 (2 brachiopods and 

 5 pelecypods) are Chemung species, not yet recorded from the typical 

 Ithaca fauna. Of these 4 pelecypods are, however, only doubtfully identi- 

 fied and the fifth, Grammy. s'm elUptica, positively only from a single 

 locality, while the two brachiopods, SchucherteUa cliemungensis and 

 Spirifer mesicostalis,^^ both occur in the Ithaca of the Chenango Talley 

 and may also be found in that of the type region. 



It is thus a significant fact that the species not holding over from the 

 Hamilton are practically all characteristic of the Ithaca fauna of the 

 typical Ithaca region or are Xaples immigrants, and we may therefore 

 conclude that these species passed eastward across the former Sherburne 

 barrier to commingle with the residual Hamilton t3rpes. 



This ability to enter the eastern waters and live side by side with the 

 unmodified Hamilton fauna, which was constantly replenished by new 

 arrivals from the Atlantic, clearly show that the Sherburne bar was no 

 longer an effective barrier, but that it became submerged early in Portage 

 time. 



In the following table the relationships of the western (t}-pical), central 

 (Chenango and Cortland counties), and eastern Ithaca faunas are shown, 

 all the doubtfully identified species being excluded. 



^^ The specimens identified by Prosser as Spirifer mesicostalis may all be forms of 

 ^. tnucronatus var. posterns, especially as the identifications from the Susquehanna 

 Valley are mostly questioned by Prosser. 



