LYMNAZID OF NORTH AMERICA. V7 
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 Lymnzeas 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 Lymnzas 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 appressa. obrussa., palustris. 
dalli. obrussa exigua. reflexa. 
humilis modicella. obrussa decampi. emarginata canadensis. 
humilis rustica. galbana. 
CLAY DEPOSITS. 
stagnalis appressa. caperata. reflexa. 
megasoma, humilis modicella. obrussa. 
