LYMNMIBJE OF NORTH AMERICA. 77 



the bibliography), in which a portion of the Ohio River is shown to 

 have drained into Lake Erie in preglacial times. Leverett also de- 

 scribes several drainage changes, besides identifying numerous pre- 

 glacial valleys in his several papers. The famous "Two Ocean Pass" 

 at the summit of the continental divide in Wyoming is a good example 

 of a route by which certain species of Lymnseas may have passed from 

 one drainage to another. Atlantic Creek flows eastward to Yellow- 

 stone River. Pacific Creek flows southward to Snake River. The two 

 creeks are connected in wet weather. (See Jordan and Kellogg, p. 309.) 

 It is to be noted that the species inhabiting the territory south of 

 the englaciated region were affected but little by the ice invasion, unless 

 it be by the added struggle for existence which the addition of the 

 northern types may have caused, and also by the more crowded con- 

 dition due to the decrease of territory. 



POST-GLACIAL AND PRE-GLACIAL SPECIES. 



The record of pre-glacial Lymnseas is obviously very imperfect. 

 With the exception of a few species recorded from the inter-glacial 

 deposits of Toronto and Cayuga Lake, all of the fossil records from 

 within the glaciated area are referrable to post-glacial times. In the 

 unglaciated regions a few records occur which are pre-glacial. One 

 species (obrussa) has been found in deposits supposed to be Pliocene 

 (Tassajora Lake bed, Alameda County, California). We have every 

 reason to believe that a large number of the existing species originated 

 prior to the Glacial Period, and the lack of records is due to the diffi- 

 culty of reaching undoubted pre-glacial deposits, which are buried 

 under the material left by the successive ice invasions. The territory 

 outside the glaciated area, as well as the driftless area of Wisconsin, 

 should produce a number of records which would doubtless prove the 

 pre-glacial existence of many recent species. Misidentifications as well 

 as references to the older composite species also render the glacial 

 records quite incomplete. Our knowledge of fossil representatives of 

 recent species may be tabulated as follows : 



SPECIES IN DEPOSITS OF ENGLACIATED AREA. 



MARL DEPOSITS. 



stagnalis apprcssa. 



obrussa. 



palustris. 



dalli. 



obrussa exigua. 



reftexa. 



humilis modicella. 



obrussa dccampi. 



cmarginaia canadensis. 



humilis rustica. 



galbana. 



CLAY DEPOSITS. 





stagnalis appressa. 



caperata. 



reflexa. 



megasoma. 



humilis modicella. 



obrussa. 



