POST-GLACIAL MOLLUSCA 667 
SANGAMON RIVER, BELOW MAHOMET 
About three-quarters of a mile below Mahomet, on the north 
bank of the Sangamon River at the first bend below the second 
bridge, a sand stratum occurs in a section of the river bank on the 
old flood plain. The stratum is about 2 inches in thickness and 
lies from 8 to 12 inches below the surface of the river terrace and 
about 5 feet above the level of the river, the latter being rather 
high for the season (July). The deposit can be traced for over a 
hundred feet in the bank. Its extent in the terrace could not be 
ascertained, but it is apparently considerable. At one place it 
disappeared beneath a large tree stump over two feet in diameter, 
indicating that the deposit antedated the present forest. The 
terrace here appears to be above the influence of the highest water, 
and it is.evident that the deposit is post-glacial and was laid down 
before the Sangamon had cut its bed to the present depth. It 
possibly represents the flood plain of an earlier Sangamon. 
It will be noted that this deposit is at a higher level than the one 
examined by Dr. Savage, and is probably younger. Of the species 
obtained from the higher level, 8 are different from those collected 
by Savage, and 4 species are included in the Savage collection that 
are not in the Baker collection. Pleurocera elevatum occurs in both 
collections and is still living in the Sangamon River. In both 
fossil deposits it is very variable in the degree of spiral ornamenta- 
tion, varying, perhaps, more than the recent shells of the Sangamon. 
Ambloxis integrum is fairly common at the higher level deposit but 
rarer at the level from which Dr. Savage coilected his specimens. 
This species lives in the Sangamon in large numbers but has not 
as long a spire as the fossil individuals. 
The surface of the flood plain of the Sangamon River near the 
first bridge below Mahomet is covered with dead, bleached shells 
of mollusks, of the same species, mostly, as those found in the 
deposit examined lower down the river, but they are mixed more or 
less with recent shells from the river and their age and stratigraphic 
relation cannot be definitely determined. No deposit comparable 
to that found in the bend of the river below Mahomet was ob- 
served here, though the river bank was carefully examined for a 
