ILLINOIS GEOLOGtCAl

SURVEY tmQAPV

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STATE OF ILLINOIS

DEPARTMENT OF REGISTRATION AND EDUCATION

Chronology and Molluscan Paleontology
of Two Post-Woodfordian Bogs
in Northeastern Illinois

A. Byron Leonard

ILLINOIS STATE GEOLOGICAL SURVEY

John C. Frye, Chief Urbana, IL 61801

CIRCULAR 487 1974

Digitized by the Internet Archive

in 2012 with funding from

University of Illinois Urbana-Champaign

http://archive.org/details/chronologymollus487leon

CHRONOLOGY AND MOLLUSCAN PALEONTOLOGY

OF TWO POST-WOODFORDIAN BOGS

IN NORTHEASTERN ILLINOIS

A. Byron Leonard

ABSTRACT

Hand -auger borings intwopost-Woodfordian basic
bogs, one northeast of Strawn in Livingston County and
one west- southwest of Batavia, Kane County, Illinois,
provided the materials for collections of fossil mollusks
from a total of 27 levels. Each collection consisted of
approximately 1,000 shells; faunal assemblages varied
from 10 to 19 species. In all, 31 species, grouped in
18 genera, were recovered from fine-grained water-laid
sediments; sediments and molluscan assemblages from
the two bogs were remarkably similar. Each of the bogs —
and their remnant lakes — occupies a basin in morainal
topography; the lakes are maintained at present by runoff
from surrounding slopes and by groundwater inflow.
Radiocarbon analyses of carbonaceous materials in the
samples range from about 9,000 years B.P. several feet
above the bottom of the sediments to about 2,000 years
B.P. at the base of the sphagnum matte that essentially
terminated the biological activity in the bog lakes. It is
estimated that the deposition in the basins began more
than 10,000 radiocarbon years B.P.

Six species, Amnioola gelida , A. leightoni ,
Gyraulus altissimus , Helisoma antrosa , Physa gyrina
hildrethiana , and Valvata tvioavinata , occur throughout
the sediments in both of the bogs; of these, three species
are branchiates and all are aquatic. Except for four spe-
cies of terrestrial habit (three of these represented by
single specimens), the entire fauna in the two bogs is
aquatic in habitat requirements. Specific data concerning
the habitat requirements of these freshwater mollusks are
lacking, but a comparison of the fossils with their living
counterparts, where available, indicates that the mollus-
can faunal assemblages in the two bog lakes lived in
clear, cool, moderately eutropic waters, basic in reaction,
sufficiently supplied with dissolved carbonates to sup-
port strong growth of molluscan shells, and characterized
by fluctuating levels.

1

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

INTRODUCTION

Available evidence points to the end of the Woodfordian Substage of the
Wisconsinan Stage of the Pleistocene at about 12,500 radiocarbon years B.P.
(Willman and Frye, 1970) . Dissipation of the last Woodfordian glaciers in the
Lake Michigan Basin left, here and there, isolated masses of glacial ice, which,
protected by mantling debris, slowly melted; in closed or poorly drained basins
in a morainal topography, these glacial ice blocks formed ponds and lakes of
various sizes and shapes. As the climate ameliorated, aquatic plants and ani-
mals colonized these small lakes, many of which terminated as peat-covered bogs ,

Two such bogs (figs. 1 and 2) are the subject of this study; one of them
is situated on the Delmar Ford farm, 1.5 miles north and 4 miles east of the vil-
lage of Strawn, in the SWi SWi SEi section 32,T.26N.,R. 8E., Livingston
County, Illinois (Strawn NE Section, localities 135-1, 135-2). The second bog
is situated about 85 miles north of the first, and about 2 3/4 miles west-south-
west of Batavia, on property of Batavia Soil Builders, Paul Wasser, president.
The hand-auger boring that provided the data for the Batavia West Section
(locality 136) is on this property in the SEi SEi section 24, T. 3 9 N. , R. 7 E. ,
Kane County, Illinois. At each of the localities, property owners were graciously
cooperative; their assistance is gratefully acknowledged.

Volo Bog, situated northwest of the city of Chicago, in Lake County,
Illinois, has been intensively studied (McComas, Kempton, and Hinkley, 1972)
geologically and hydrologically. In addition to describing the portion of the bog
with a pH in the acid range, the authors (who incorrectly refer to Volo Bog as
unique in Illinois because "no other bogs in Illinois have open-water ponds")
describe a portion in which Typha and sedges produce an alkaline sediment, but
they make no reference to fossil shells in these basic deposits. Their study of
the time framework of the development of the Volo Bog corresponds well with the
available dates for the Strawn NE and Batavia W bogs; especially interesting
perhaps is the date they obtained from the base of about 5 feet of peat, 2, 100 ±
200 years B.P. (ISGS-49) , which is almost identical with the date obtained from
a similar peat deposit at Batavia W (1,870 ± 200 B.P. , ISGS-134) .

Baker (1910) investigated the Skokie Bog with special reference to the
molluscan faunas living there at the time; he did not explore the sediments in
the bog for fossil mollusks.

STRAWN NORTHEAST LOCALITY

The two hand-auger holes bored at the Strawn Northeast locality (135-1,
135-2, fig. 1) were made at the water's edge of a small lake maintained by a
brook that enters the bog from the Paxton Moraine to the east; a colony of
beavers regulates the water level of the lake by a dam which these animals keep
in good repair. The depression in which the fossiliferous sediments have accu-
mulated (fig. 1) seems to have been occupied by a glacial ice remnant in a
morainal topography. The present lake is about 200 by 400 yards, but deposits
indicate that the lake was originally at least twice this size. Only a 12 to 18
inch layer of peat covered the bog; this was removed by operations begun in

POST-WOODFORDIAN MOLLUSKS

Fig. 1 - Excerpt from Sibley, Illinois, 15 -minute quadrangle map, contour interval 10 feet,
showing the location of the Strawn NE bog, and the position of the two hand-auger
borings, localities 135-1 and 135-2. The high ground to the east of the bog lake
is the front of the Paxton Moraine. A canal provides drainage for the basin, but a
colony of beavers maintains a dam that regulates the level of the lake.

1934 to recover fossilferous, highly organic marl for use as a substitute for agri-
cultural lime and fertilizer. These operations have been virtually suspended.

The cuttings from the two auger holes provided the samples upon which
the measured section is based and from which the molluscan faunas were recovered
Auger hole 131-1, according to judgments based upon local topography, is near
the southern shore of the original basin; here glacial outwash and/or till was
encountered at a depth of 14 feet. Auger hole 135-2 was bored about 200 yards
north, on the opposite shoreline of the present lake; it is judged to be near the

1 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

center of the original basin. Sediments were penetrated to a depth of 20 feet, at
which depth sidewall collapse prevented further augering; till was not encountered
but the paucity of molluscan fossils, increase in percentage of clay, and occur-
rence of pebbles indicated the nearness of the base of the bog deposits.

Strawn Northeast Section

Strawn NE Section, measured in auger holes in the SW^SW^SE^
sec. 32, T. 28 N. , R. 8 E., Livingston County, Illinois.

Thickness
(feet)

5. Black, highly fossilif erous organic muck, partially
exposed in wave-cut bank; contains some silt and
very fine sand. Radiocarbon date from near base
of bed, 2640 ± 75 B.P. (ISGS-161) 2.0

4. Black organic muck; fine sand, silt, clay, much
organic debris, and abundant molluscan fossils.
Radiocarbon dates from near base of interval,
2330 ± 75 B.P. (ISGS-162A) and 2370 ± 100 B.P.
(ISGS-162B) 4.0

3. Dark gray marl (lighter gray near base of unit);
fine sand, silt, clay, much organic debris, and
many molluscan fossils. Radiocarbon date from
near base of interval, 7760 ± 84 B.P. (ISGS-164) . . 4.0

2. Gray marl; fine to medium sand, silt, and clay;

plastic when wet; contains a few pebbles to ^ inch
diameter; abundant plant remains and fossil mol-
lusks declining in abundance toward base of unit.
Radiocarbon date from near base of interval,
8940 ± 80 B.P. (ISGS-167) (Coleman, 1974) 4.0

1. Gray sand; silt and clay mixed with numerous

pebbles; contains a few mollusks and plant remains

in upper part; sterile toward base in glacial till

and/or consolidated outwash. Organic materials

too sparse for radiocarbon date 3.0

Total 17.0

BATAVIA WEST LOCALITY

The Batavia West Section is also described from a hand-auger boring; the
boring is situated along the northwest border of a depression that extends for more
than a mile in a southwesterly-northeasterly direction. All of the depression is
a peat-covered bog, except for Nelson Lake, which occupies part of the

POST-WOODFORDIAN MOLLUSKS

Fig. 2 - Excerpts from Aurora North and Sugar Grove, Illinois, 7.5-minute quadrangle maps,
contour interval 10 feet with dotted 5-foot contour, showing the Batavia W bog and
the location of the hand-auger boring, locality 136. The basin is situated in a
re-entrant of the St. Charles Moraine; it is partially drained by a channelized
brook, "Lake Run."

southern end of the depression (fig. 2) . The bog, at least in the northern part,
bears a layer of peat to a thickness of 4 to 5 feet; this is being harvested
commercially by Batavia Soil Builders as soil conditioner. The hand-auger
boring penetrated nearly 5 feet of peat; the hole terminated in nonfossiliferous,
unctious clay, where it had to be abandoned because of sidewall collapse.

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

Batavia West Section

Batavia W Section, measured in auger hole situated in the SE^SE^
sec. 24, T. 39 N. , R. 7 E., about 2.75 miles WSW Batavia,
Kane County, Illinois.

8. Dark brown peat; no fossil shells except in lower
few inches. Radiocarbon date from lowermost 2
inches of interval, 1750 ± 100 B.P. (ISGS-131)
(Coleman, 1974)

7. Gray-tan silt, clay, and marl; contains

abundant fossil mollusks

6. Gray-tan marl with abundant fossil mollusks

and plant remains

5. Gray-tan marl and clay; plastic and wet;

abundantly fossilif erous

4. Dark gray marl and silt, some undecayed peat;

fossilif erous , very wet, almost semifluid . . . .

3. Dark gray fossilif erous silt, with small amount
of plant material; contains small masses of
sterile gray clay

2. Dark gray, sandy, nonfossilif erous clay;

plastic and very wet

1. Base in sterile gray clay, no pebbles

Total

Thickness
(feet)

4.5

0.5
2.8
2.5
2.1

3.3

1.6

0.6

17.9

Generalities

Augering was done with a sandspoon because of the almost semifluid
condition of the bog sediments. Samples were removed from the spoon and col-
lected in plastic bags, each of which was labeled with appropriate data. The
collected samples were subsequently washed over wire screens; the residue was
dried and the fossil shells were sorted from the residue. Although no attempt
was made to collect samples of uniform size, most of them contained about
1,000 shells. A few fossil seeds were recovered; all the samples, except the
sterile clay and sand at the base of the borings, contained many ostracods and
even more diatoms.

It is judged that most of the fossil shells at each of the two localities
collected on the bottom of a quiet lake; evidence for this includes the apparent
lack of sorting of fine sediments and the fact that in all borings and at nearly
all levels many small pelecypods were recovered with the two valves still united,
despite the operations of boring and the later treatment of the samples. However,

POST-WOODFORDIAN MOLLUSKS /

as discussed below, there is evidence that some shells reached the deposit
from outside the lakes.

The original basins in which the two bogs developed are judged to be as
old as 10,000 to 12,000 years, inasmuch as the lowermost finite date of about
9,000 radiocarbon years B.P. (Coleman, 1974) is approximately 5 feet above
the base of the bog sediments. A reasonable extrapolation based on the rate of
dated sedimentation produces this approximation.

SYSTEMATIC ACCOUNT OF MOLLUSCAN SPECIES

The molluscan fauna is listed here in systematic order, and reference is
made to the original description of each taxon, to a standard work using the
present name combination, and to a description and illustration of each kind
of shell.

Class Pelecypoda

Order Prionodesmacea
Family Unionidae

Fragments recognizable as those of unionid mussel shells were recovered
from time to time in auger samples from each of the three holes augered, but
none of these was large enough to do more than relate them to the pelecypod
family. No hinge teeth were recovered.

Order Teleodesmacea
Family Sphaeriidae

Genus Sphaeriwn Scopoli 1777

H. B. Herrington, in his revision of the North American Sphaeriidae
(1962), recognized 35 kinds of these animals, 31 of them native to North
America and 4 kinds judged to have been introduced from Europe. The sphae-
riids were distributed by Herrington among 3 genera — Sphaeriwn , having 12
recognized species; Pisidiwn , having 22 species; and the New World genus
Eupera , which is represented in North America by the single species oubensis .
Burch(1972), in his illustrated key to the Sphaeriidae of North America, added
4 species of Pisidiwn to the list recognized by Herrington.

Sphaeriwn seouris (Prime) 18-51

Cyclas seouris Prime 1851, Boston Soc. Nat. Hist. Proc, v. 4, p. 160
Sphaeriwn seouris Prime 1865, Monogr. Amer. Corbiculidae, p. 49.
Sphaeriwn seouris (Prime), Herrington 1962, Revision Sphaeriidae
North America, p. 26, pi. 1, fig. 2.

The shell of S. seouris is small for the genus, usually not more than
8 mm in length; the beak is usually, but not invariably, calyculate. The species
occurs in seven of the faunal assemblages; it is distributed through various
levels and at each of the three localities (fig. 3) . In no assemblage were the
shells numerous; in fact they should be categorized as rare since numbers of
valves ranged from 1 to 7 in any particular assemblage.

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

^"^>^^ LOCALITIES AND LEVELS
^"^^^ OF SAMPLES

MOLLUSCAN SPECIES ^"""^^^

STRAWN NE, No. 135-1
(intervals in feet)

CM

1

O

i

CM

vD
1

00

1

o

1

00

CM

1

O

i-H

1

CN

Amnioola gelida F. C. Baker

•

Amnioola leightoni F. C. Baker

•

Gyraulus altissimus (F. C. Baker)

•

Eelisoma antrosa (Conrad)

•

Physa gyrina hila^rethiana Lea

•

Valvata trioarinata (Say)

•

Promenetus wnbilicatellus (Cockerell)

•

Pisidiwn nitidum Jenyns

•

Promenetus exaouous (Say)

•

Pisidiwn compression Prime

•

Sphaerium simile (Say)

•

Ferrissia parallela (Haldeman)

Lymnaea (Fossaria) parva Lea

•

Lymnaea (Fossaria) obrussa Say

Eelisoma oampanulatum (Say)

Valvata lewisi Currier

•

•

•

Sphaerium seouris (Prime)

•

Lymnaea (Stagnioola) lanoeata Gould

•

Eelisoma trivolvis (Say)

•

Lymnaea (Fossaria) dalli F. C. Baker

•

Armiger exigua Leonard

•

Planorbula armiger a (Say)

•

Suceinea (Oxyloma) retusa Lea

•

•

Aplexa hypnorum (Linne)

•

Caryohium exile Lea

•

Eelioodisous parallelus (Say)

•

Lymnaea (Stagnioola) oaperata Say

•

Lymnaea (Stagnioola) palustris (Muller)

Pseudosuooinea columella (Say)

•

Sphaerium fabale (Prime)

Vertigo modesta (Say)

Total number of species

19

12

10

11

17

13

13

Fig. 3 - Chart showing occurrence
at three localities, 135-1

and d
, 135-

istribution of
2, Strawn NE,

31 species
Livingston

POST-WOODFORDIAN MOLLUSKS

STRAWN NE, No. 135-2
(intervals in inches)

BATAVIA W, No. 136
(intervals in inches)

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00
CM

1

00

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1

00
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CM

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1
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m
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1

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CM

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1

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1— 1

CM

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1

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1

cr*

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-d-

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<d-

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1

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00

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1

<r

H

S3-

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CM

1

CO
H

•

•

•

m

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

12

14

9

14

13

12

11

13

13

12

12

15

11

13

15

13

16

16

16

15

of mollusks obtained from hand-auger cuttings from 27 levels
County, and 136, Batavia W, Kane County, Illinois.

10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

Sphaerium simile (Say) 1816

Cyolas similis Say 1816, Nicholson's Encyc . , v. 2, pi. 1, fig. 9.
Sphaerium simile (Say), Burch 1972, Biota Freshwater Ecosystems,
Washington, p. 6, 10, fig. 5b.

The specific name of this common sphaeriid mussel has long been a
matter of controversy; the problem was discussed in some detail by F. C. Baker
(1928, p. 316). Herrington considered the problem and indicated that simile
is not a synonym of sulcatum. However, Burch (1972), who worked with Herring-
ton, does not recognize sulcatum, but uses Herrington' s figure of sulcatum to
illustrate simile. Since the species is widely distributed in North America, it
probably is a native element of the sphaeriid complex, distinct from sulcatum.

The large shell of S. simile occurs in 12 of the 27 faunal assemblages
considered (fig. 3), but in none are the valves numerous. They vary from frag-
ments to eight complete valves. At localities 135-1 and 135-2, simile does
not occur in the uppermost 5 to 6 feet of the deposits, but at the Batavia locality,
136, simile ranges throughout the section.

Sphaerium fabale (Prime) 1851

Cyclas fabalis Prime 1851, Boston Soc. Nat. Hist. Proc . , v. 4, p. 159.
Sphaerium fabale (Prime), Herrington 1962, Revision Sphaeriidae
N. America, p. 18, pi. 2, fig. 3.

A single valve recovered from the 12-foot level of locality 136 is the
only evidence of this species in the bogs under study. It seems unlikely that
the animal lived in the lake itself, at least where the auger boring was made.
This valve is somewhat smaller than those described by Herrington, but it seems
to be a mature shell.

S. fabale is distributed in North America from Vermont to the Mississippi
River; it has been reported in Canada from southern Ontario.

Genus Pisidium Pfeiffer 1821

The shells of the genus Pisidium are small, rounded or ovoid to cuneiform .
seldom much over 6 mm in length. They differ frorn those of Sphaerium inasmuch
as the beaks are posterior. The species recognized in North America are divided
arbitrarily into two groups: four "large" species and 21 "small" species. The
two species of Pisidium considered here belong to the "small" group.

Pisidium compressum Prime 1851

Pisidium compressum Prime 1851, Boston Soc. Nat. Hist. Proc, v. 4,
p. 164.

Pisidium compressum Prime, Herrington 1962, Revision Sphaeriidae
N. America, p. 35, pi. 5, fig. 2; pi. 7, fig. 14.

The shell of P. compressum is 3.5 to 4.0 mm long, robust for its size,
and characteristically trianguloid in shape. In our collections, it occurs in 21
of the 27 faunal assemblages and is numerous in every occurrence; it is absent
from the uppermost 4 to 6 feet of the auger samples at localities 135-1 and
135-2, but occurs abundantly throughout the several levels of locality 136
(fig. 3) . A few united pairs of valves were observed at each of the localities
studied.

POST-WOODFORDIAN MOLLUSKS 11

P. compression is distributed widely in North America, from northern
Canada to California and Mexico.

Pisidiwn nitidwn Jenyns 1832

Pisidiwn nitidwn Jenyns 1832, Cambridge Philos. Soc . , v. 4, p. 304,
pi. 20, figs. 7-8.

Pisidiwn nitidwn Jenyns, Herrington 1962, Revision Sphaeriidae
N. America, p. 45, pi. 5, fig. 6, pi. 7, fig. 17.

Pisidiwn nitidwn is a small pelecypod, usually less than 3 mm in length;
it has a generally rounded form, inconspicuous beaks, and very fine striations
on the shell. It is distributed widely in Europe as well as in North America.

It occurs in 24 of the 2 7 faunal assemblages studied (fig. 3), but at
none of them were the valves numerous. In many of the assemblages, pairs of
valves occurred intact, indicating that the species is indigenous to the deposits
even though not abundant in them. Some of the shells possess the characters
referred to as the "form pauperoulwn, " which does not seem to be a valid
taxonomic entity.

Class Gastropoda

Subclass Streptoneura
Order Ctenobranchiata
Family Valvatidae

Genus Valvata Miiller 17 74

The genus Valvata encompasses a few species of small, spiral, turbinate
gastropods that carry on gaseous exchange by means of gills; they therefore re-
quire permanent water, reasonably free of silt. There are probably no more than
three or four species of this genus, which is unique to North America, but many
"forms" of little or no taxonomic value have been described (Baker, 1928, p. 7-
32; La Rocque, 1968, p. 358-368).

Valvata lewisi Currier 1868

Valvata lewisi Currier 1868, Kent Sci. Inst. Misc. Pub. No. 1, p. 9.
Valvata lewisi Currier, La Rocque 1968, Ohio Div. Geol. Surv. Bull. 62,
pt. 3, p. 360, fig. 211.

Valvata lewisi is a small gastropod with a spiral, turbinate shell bearing
regular striae that have been compared to the winding of a thread on a spool.
The shells are about 4.5 mm in diameter, and are 3 or more mm high. In this
study, V. lewisi occurred in only 8 of the 27 faunal assemblages, and in
these the species was represented by no more than 1 to 4 shells. Occurrences
were limited to the deeper levels of localities 135-1 and 135-2; no shells were
found in the abundant faunal assemblages at locality 136 (fig. 3).

This species is distributed across Canada and the northern part of the
United States; it has been reported living in La Salle County, Illinois (Baker,
1928, p. 28).

Valvata tricarinata (Say) 1817

Cyclostoma tricarinata Say 1817, Acad. Nat. Sci. Philadelphia Jour.,
v. 1, p. 13.

Valvata tricarinata (Say), F. C. Baker 1928, Freshwater Moll.
Wisconsin, pt. 1, p. 11, pi. 1, figs. 1-3.

12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

Valvata trioarinata possesses a shell shaped much like that of V. lewisi,
but the whorls are tricarinate, although the development of the carinae varies
greatly. This variation has led to the publication of many trinomial names, which are
essentially meaningless since many variations of the shell ornamentation occur
in the same population of the species. Furthermore, there is no correlation be-
tween variation in shell carinae and internal anatomy. If writers feel constrained
to refer to the variations in the development of the carinae, La Rocque has sug-
gested a means of doing this (1968, p. 368) which certainly is better than
trinomial designations.

V. trioarinata forms a conspicuous element of each of the faunal
assemblages studied; it occurs in each of the 27 assemblages, and is in every
one of them the most numerously represented. It is present in the bottom samples
at each locality and was therefore one of the first species to colonize these bogs
as the original glacial lakes formed by meltwaters and by runoff from the
surrounding moraines.

The species is widely distributed in North America east of the Rocky
Mountains, and was a conspicuous animal in Pleistocene fossil assemblages
on the Great Plains; it no longer lives there south of the Sand Hills area in
Nebraska .

Family Amnicolidae
Genus Amnioola Gould and Haldeman 1841

The genus Amnioola is composed of small branchiate gastropods having
tightly coiled spiral shells; the height of the shells is rarely more than 6 mm
and in most species is less. Although the genus is distributed primarily in
northeastern North America, living species extend as far southwest as eastern
Kansas. However, members of the genus thrive best in clear, cold waters.
Several genera are recognized in the family Amnicolidae (Berry, 1943), but only
one of them occurs in the bog faunas studied here.

Amnioola gelida F. C. Baker 1921

Amnioola lustrioa gelida F. C. Baker 1921, Nautilus, v. 35, p. 22.
Amnioola gelida?. C. Baker 1928, Freshwater Moll. Wisconsin, pt. 1,
p. 110, pi. 6, figs. 19-23.

Although not as numerous as V. trioarinata in these collections,
Amnioola gelidaoccurs abundantly in every one of the faunal assemblages
studied. A. gelida is known only as a fossil that has been reported in Ohio
and in Illinois. The shells are high spiraled. Sexual dimorphism seems ap-
parent among them; the shells presumed to have contained male animals
possess a much broader ultimate whorl than do the presumptive females. This
is a common phenomenon among amnicolids, produced by the necessity of accom-
modating the large male intromittent organ (verge) . A. gelida was also an early
colonizer of the glacial lakes; the shells occur in the lowermost samples that
contain any fossils at all.

Amnioola leightoni F. C. Baker 1920

Amnioola winkleyi leightoni F. C. Baker 1920, Nautilus, v. 33, p. 125.
Amnioola leightoni F. C. Baker 1921, Nautilus, v. 35, p. 23.
Amnioola leightoni F. C. Baker 1928, Freshwater Moll. Wisconsin,
pt. 1, p. 120, pi. 6, figs. 34-39.

POST-WOODFORDIAN MOLLUSKS 13

The shells of this species have a characteristic bulbous shape produced
by the large ultimate whorl, which differs little between the sexes. The shells
are spiral, turbinate, and relatively small, the height being no more than 5 mm
in adult specimens. Like A. gelida, A. leightoni is presumed to be an extinct
species, which has been reported from Pleistocene marl deposits in Illinois,
Ohio, Indiana, and elsewhere in nearby states.

Amnioola leightoni occurs in every one of the 2 7 faunal assemblages
studied and forms abundant populations in each of them. It was one of the
earliest colonizers of these bogs, and occurs in the lowermost fossiliferous
samples at each locality.

Subclass Euthyneura
Order Pulmonata
Suborder Basommatophora
Family Lymnaeidae

Genus Lymnaea Lamarck 1799

The lymnaeids are pulmonate aquatic gastropods, widely distributed over
the world, especially in the northern hemisphere. The animals are extremely
sensitive to local environmental conditions and to fluctuations in them, which
has led to great taxonomic confusion and needless proliferation of names.
Baker (1911) subdivided the genus Lymnaea into several genera; this subdivision
has been widely, although not universally, accepted. The lymnaeids seem
rather to be represented worldwide as a small number of highly variable species,
as pointed out and so ably defended by Hubendick (1951).

In this work all taxa of lymnaeids are listed in the genus Lymnaea, as
seems proper, especially in the case of fossil shells, but the names of the
Bakerian genera are inserted as subgenera in order to reflect wide usage among
students of the family Lymnaeidae.

Aquatic pulmonate gastropods possess no gills; they mediate gaseous
exchange through a pocket in the mantle, or through the general body surface.
Active lymnaeids have been found at great depths, indicating their independence
of free gaseous oxygen.

Although several taxa of lymnaeids occur in the two bogs under investi-
gation, the shells are nowhere numerous and should be classed everywhere in
these bogs as rare.

Lymnaea {Stagniaola) oaperata Say 1829

Lymneus caperatus Say 1829, New Harmony Disseminator, v. 2, p. 230.
Stagniaola oaperata (Say), F. C. Baker 1928, Freshwater Moll.
Wisconsin, pt. 1, p. 260, pi. 18, figs. 43-47.

L. oaperata is represented in these collections by a single example
(fig. 3), which was probably transported into the bog from small brooks that
flowed into it. It cannot be said to have recognizable significance in this study.

Lymnaea {Stagnioola) lanoeata Gould 1848

Limnaea lanoeata Gould 1848, Boston Soc. Nat. Hist. Proc, v. 3, p. 64,
Stagnioola lanoeata (Gould), F. C. Baker 1928, Freshwater Moll.
Wisconsin, pt. 1, p. 228, pi. 14, figs. 12-15.

L. lanoeata occurs in only 6 of the 21 faunal assemblages, and in each
of these is represented by only one to three shells. The occurrence of the

14 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

species in the two bogs (fig. 3) seems to have no correlation with the depth of
the sample. The shell is elongate -cylindrical and may vary in length from 20
to 30 mm. However, the few shells collected in these deposits are either im-
mature or broken, and are much smaller than the typical dimensions. L. lanoeata
has been reported from southern Ontario to Ohio and Wisconsin.

L. lanoeata, sparsely and erratically distributed in the Strawn and
Batavia bogs, certainly cannot be considered a significant element of the faunal
assemblages .

Lymnaea {Stagnioola) palustris (Miiller) 1774

Bucoinum palustre Miiller 1774, Verm. Terr, et Fluv. Hist., p. 131.
Stagnioola palustris (Miiller), La Rocque 1968, Ohio Div. Geol. Surv.
Bull. 62, pt. 3, p. 443, fig. 294.

A single shell of L. palustris was taken from the uppermost sample at
locality 135-2 (fig. 3). The species is one adapted for living in a small lake
habitat, but the single shell makes it doubtful that established populations
inhabited the lake at the Strawn locality. L. palustris has an elongate spiral
shell usually more than 20 mm high.

Lymnaea {Fossaria) dalli F. C. Baker 1906

Lymnaea dalli F. C. Baker 1906, Illinois Lab. Nat. Hist. Bull., v. 7,
p. 104.

Fossaria dalli (F. C. Baker), F. C. Baker 1928, Freshwater Moll .
Wisconsin, pt. 1, p. 288, pi. 16, fig. 11.

L. dalli is a tiny gastropod with a typical lymnaeid shell no more than
about 4 mm in length. The living species occurs more often near water than in
it, on mud and debris near the water's edge. It is widely distributed in North
America, probably more so than records indicate because its small size leads
to its being overlooked. It occurs in five of the faunal assemblages (fig. 3),
in fair numbers in some collections. In spite of this fact, it is doubtful that
the animals lived where the shells were found, because L. dalli is rarely if
ever found far from shoreline habitats.

Lymnaea {Fossaria) obrussa Say 1825

Lymneus obrussus Say 1825, Acad. Nat. Sci. Philadelphia Jour., v. 5,
p. 123.

Fossaria obrussa (Say), F. C. Baker 1928, Freshwater Moll . Wisconsin,
pt. 1, p. 239, pi. 18, figs. 14-24.

L. obrussa is a rather small lymnaeid gastropod, the shells varying from
9 to 11 mm in length; it is larger than L. parva and the ultimate whorl is more
swollen than it is in L. parva. L. obrussa was not recovered from any level at
locality 135-1. It occurs sparingly in three faunal assemblages at locality
135-2, but is found at every level among the samples from locality 136 (fig. 3) .
Even at the Batavia bog (136) L. obrussa does not occur in abundant numbers,
although its regular occurrence would indicate it as a part of the indigenous
molluscan population.

Lymnaea {Fossaria) parva Lea 1841

Lymnaea parva Lea 1841, Amer. Philos . Soc. Proc, v. 2, p. 33.

POST-WOODFORDIAN MOLLUSKS 15

Fossaria parva (Lea), F. C. Baker 1928, Freshwater Moll. Wisconsin,
pt. 1, p. 285, pi. 18, figs. 1-5.

Lymnaea parva is a small gastropod, the shell varying from 4.5 to 9 mm
in length. The shell is slender, and the whorls are rounded. Like L. dalli
it is more often found out of the water than in it; Baker (1928, p. 287) notes that
L. parva "is more prone to leave the water than any other member of the family."
This small gastropod occurs throughout the two bogs studied and is found in 11
of the 27 faunal assemblages; it is slightly more numerous in the assemblages
recovered at the Batavia site (locality 136) . Although it does not occur at any
level in abundance, its distribution throughout the various sampled levels indi-
cates that it was probably a part of the indigenous molluscan fauna.

Genus Pseudosuooinea F. C. Baker 1908

Baker removed the species Lymnaea columella of Say 1817 from the genus
Lymnaea and established the genus Pseudosuooinea on the basis of the peculiar
succineiform shape of the shell and certain anatomical characteristics. This
change has not met with universal approval, but most authors have followed
Baker.

Pseudosuooinea oolumella (Say) 1817

Lymnaea oolumella Say 1817, Acad. Nat. Sci. Philadelphia Jour., v. 1,
p. 14.

Pseudosuooinea oolumella (Say), F. C. Baker 1928, Freshwater Moll.
Wisconsin, pt. 1, p. 272, pi. 10, figs. 9-12, 20.

The shell of P. oolumella, as the name suggests, greatly resembles the
shells of Suooinea , except that it has more whorls than the members of the
latter genus. Adults are as long as 12 mm, but the single shell recovered in this
study, from the 10- to 12-foot level of locality 135-1, is much smaller although
it has nearly five whorls. The environment at the bog lake seems appropriate,
but the single shell recovered indicates that P. oolumella was not a significant
element of the molluscan fauna. P. oolumella is widely distributed in North
America. The genus Pseudosuooinea extends through Central and South America.

Family Planorbidae

Genus Gyraulus Agassiz 1837

The genus Gyraulus is composed of small planorbid gastropods which
live as aquatic pulmonates, principally on vegetation, in ponds, lakes, or the
quiet parts of streams. They are inconspicuous because of their small size and
dark color, but may often exist in abundant populations. Species of Gyraulus
are persistent and often conspicuous elements in fossil molluscan assemblages
from water-laid sediments. The genus is worldwide in distribution. La Rocque
(1968, p. 483) points out means of distinguishing members of the genus Gyraulus
from others with which they might be confused.

Gyraulus altissimus (F. C. Baker) 1919

Planorbis altissimus F. C. Baker 1919, Nautilus, v. 32, p. 95, pi. 7,
figs. 7-10.

Gyraulus altissimus (F. C. Baker), F. C. Baker 1928, Freshwater Moll.
Wisconsin, pt. 1, p. 382, pi. 22, figs. 10-17.

16 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

Like many other species of Gyraulus , altissimus is characterized by
Zh to 4 rapidly increasing whorls and the lack of a distinctive umbilicus. It is
recognized by the fact that the periphery of the shell is situated in a ventral
position; the ventral whorl is not flattened as in G. parvus. G. altissimus is the
only species of the genus recognized in these faunal assemblages (fig. 3),
where it occurs in great numbers at every level studied, but many shells tend
toward G. arotious and some tend to approach parvus. The dense populations of
G. altissimus in all faunal assemblages make this species a significant element
of the molluscan faunas in these bog lakes.

Genus Armiger Hartman 1840

There seems little to justify this genus as distinct from Gyraulus except
the shell sculpture; the soft anatomy is similar to that of species of Gyraulus.
Granting validity to the genus, species of Armiger are found in North America,
Europe, and Asia.

Armiger exigua Leonard 1972

Armiger exigua Leonard 1972, Nautilus, v. 85, p. 81, figs. 1-2 and 1-3.

This tiny gastropod closely resembles A. crista, but is much smaller.
It has the same number of whorls; the nuclear whorl is granular rather than
striate; there is no indication of spiral striation; and the last whorl does not
descend toward the aperture, which is roundly oval in shape. It occurs sparingly
in five of the faunal assemblages (fig. 3), none of them in the Batavia locality
(136). Nothing is known, of course, of the ecological requirements of this mi-
nute gastropod beyond that which can be deduced from its associates. The
species was described from shells found in Petersburg Silt of early Illinoian age,
Henry County, Illinois. It has been found in late Woodfordian sediments in
eastern New Mexico.

Genus Helisoma Swain son 1840

Species of Helisoma are among the large gastropods of the family
Planorbidae. Shells are robust, the cross striation is typically coarse, and
spiral striation may be conspicuous.

Helisoma antrosa (Conrad) 1834

Planorbis antrosus Conrad 1834, Amer. Jour. Sci., 1st ser., v. 25, p. 343
Helisoma antrosa (Conrad), F. C. Baker 1928, Freshwater Moll.
Wisconsin, pt. 1, p. 317, pi. 19, figs. 8-15.

Helisoma antrosa is another of the six species that occur in every faunal
assemblage in the localities studied (fig. 3) although population numbers are
lower than those of any other of the persistent species except Physa gyrina
hildrethiana. Nevertheless, this species forms a conspicuous element of the
total molluscan assemblages.

Many, but not all, of the examples collected have the spiral striations
that characterize a described form {striata F. C. Baker) that supposedly
characterizes Pleistocene populations of H. antrosa. It seems unlikely that the
"striata" modification of sculpture has taxonomic significance.

The aperture of many Helisoma antrosa specimens is distended in a bell-
like form resembling H. campanulatum , but the spires of the two species are
entirely different; that of H. antrosa forms a deep, smooth-sided funnel, while

POST-WOODFORDIAN MOLLUSKS 17

that of H. oampanulatum is flatly coiled. Living H. antrosa is widely distributed
in North America, but populations are especially numerous east of the Missis-
sippi River.

Helisoma trivolvis (Say) 1817

Planorbis trivolvis Say 1817, Nicholson's Encyclopedia, Amer. Ed.,
v. 2, pi. 2, fig. 2 (no pagination) .

Helisoma trivolvis (Say), F. C. Baker 1928, Freshwater Moll. Wisconsin,
pt. 1, p. 330, pi. 20, figs. 1-13, 22, 23.

The shells of this large pulmonate aquatic gastropod may exceed 3 0 mm
in diameter. It is widespread in North America, except in the western mountains,
where it occurs less frequently. In the Pacific area, populations previously
referred to trivolvis are often given other names. H. trivolvis is not common
in the collections from the two bogs under study, and those populations that do
occur are obviously late colonizers (fig. 3) since shells occur only in the upper-
most 4 to 6 feet at localities 135-1 and 135-2. No shells of H. trivolvis were
recovered at the Batavia bog (locality 136).

Helisoma oampanulatum (Say) 1821

Planorbis oampanulatus Say 1821, Acad. Nat. Sci. Phila . Jour. , v. 2, p. 166.
Helisoma oampanulatum (Say), F. C. Baker 1928, Freshwater Moll.
Wisconsin, pt. 1, p. 345, pi. 21, figs. 1, 2, 4, 5, 8, 9, 13, 14.

H. oampanulatum is about the same size asH. antrosa , that is, the shell
is 10 to 15 mm in diameter; the aperture is expanded in a bell-like fashion,
hence the name. Superficially it resembles H. antrosa; its distinguishing
characters have been noted.

H. oampanulatum occurs infrequently in the assemblages from localities
135-1 and 135-2, but is present in every one of the seven faunal collections
from the Batavia bog, locality 136 (fig. 3). The living species is distributed
largely in northeastern United States, but records exist for several widely
scattered localities in Canada. It does not seem to occur south of Illinois and
Ohio.

Genus Planorbula Haldeman 1842

The shells of the pulmonate aquatic gastropods of this genus are typically
planorbid, the whorls are tightly wound, and the lip is more or less thickened
within. The truly characteristic feature of these shells, however, is the persistent
set of six dentiform lamellae a short distance back from the aperture. The inter-
nal anatomy is similar to that of Gyraulus. The genus occurs in North America
and in Africa .

Planorbula armigera (Say) 1818

Planorbis armigerus Say 1818, Acad. Nat. Sci. Phila. Jour., v. 2, p. 164
Planorbula armigera (Say), F. C. Baker 1928, Freshwater Moll. Wisconsin,
pt. 1, p. 355, pi. 8, figs. 27-30.

Planorbula armigera displays the characters of the genus; the planorbid
shell is about 7 mm in diameter. According to Baker (1928, p. 359), it is dis-
stributed from New England and Great Slave Lake to Nebraska and south to
Georgia and Louisiana. It is widespread as a Pleistocene fossil.

18 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

In the bog sediments under study here, it is erratically distributed among
five faunal assemblages at the Strawn locality (135-1, 135-2), but is absent
from the Batavia assemblages, locality 136 (fig. 3). Even where it does occur
in these sediments, it is rarely represented by more than one or two shells.

Genus Promenetus F. C. Baker 1935

F. C. Baker erected the genus Promenetus (Nautilus, 49:48) using
Planorbis exaouous Say as the type; later (1945) he recognized five species and
several varieties of these — which, of course, he referred to as subspecies — by
the form in which the so-called varietal names were used. The shells of species
in this genus are lenticular, with rapidly increasing whorls, and often with a
carina te periphery.

Promenetus exaouous (Say) 1821

Planorbis exaouous Say 1821, Acad. Nat. Sci. Philadelphia Jour., v. 2,
p. 168.

Promenetus exaouous (Say), F. C. Baker 1945, Molluscan Family Planor-
bidae, p. 182, pi. 41, figs. 1-10.

P. exaouous is distributed in North America east of the Rockies from
Canada to the mountains of New Mexico; it is absent from the central and
southern High Plains except as a Pleistocene fossil.

Promenetus umbilioatellus (Cockerell) 1887

Planorbis umbilioatellus Cockerell 1887, Conchologist's Exchange,
v. 2, p. 68.

Promenetus umbilioatellus (Cockerell), F. C. Baker 1945, Molluscan
Family Planorbidae, p. 182, pi. 43, figs. 1-12.

P. umbilioatellus was for many years considered as belonging in the
genus Gyraulus , but anatomical studies demonstrated its relation to Promenetus.
This species is almost universally represented in the two bogs under study here,
being absent from only two faunal assemblages (fig. 3) . Never present in abun-
dant populations, P. umbilioatellus is, nevertheless, represented in at least
moderate numbers where it occurs. It is, obviously, an indigenous element of
the total faunal assemblages of the two bogs.

P. umbilioatellus lives today in northern United States and southern
Canada, and in high elevations in the Rockies as far south as New Mexico.

Family Ancylidae

Genus Ferrissia Walker 1903

Shells of the gastropods of the genus Ferrissia are patelliform, or
limpetlike; the animals creep about on leaves, stems of reeds and sedges, aban-
doned shells of unionids, old bottles, and other smooth surfaces in quiet waters.
The shells are extremely simple and do not always reflect complexities that may
be present in the organization of the animal itself. The genus is worldwide in
distribution, but lacking in many places, such as, for example, Europe and
Latin America.

Ferrissia parallela (Haldeman) 1841

Anoylus parallelus Haldeman 1841, Monograph Limniades N. Amer.,
pt. 2, p. 3.

POST-WOODFORDIAN MOLLUSKS 19

Ferrissia parallela (Haldeman) , La Rocque 1968, Ohio Div. Geol. Surv.
Bull. 62, pt. 3, p. 521, fig. 375.

The shells of F. parallela are so named because the sides of the patel-
liform shell are straight and nearly parallel, although they widen somewhat
anteriorly; the apex is turned slightly to the right and is situated slightly anter-
ior of center. The shells are thin and delicate and easily broken, but many were
recovered from auger cuttings in the study of the two bogs. Although there was
some variation in the precise form of the shells recovered, it seems best to
refer all of them to F. parallela.

This little ancylid, rarely more than 5 mm in length, is distributed
somewhat erratically through the faunal assemblages studied; how much the
apparent occurrence (or lack of it) depends upon the hazards of collecting such
delicate shells is not known. They thrive in a pond environment, especially
if large vascular plants such as Typha are present.

F. C. Baker (1928, p. 397) quotes Walker as saying that F. parallela
is a northern species distributed from southern Canada to northern Ohio and
Indiana. A few records exist for southern states, however; and it is widely
distributed as a Pleistocene fossil in the Great Plains.

Family Physidae

Genus Physa Draparnaud 1801

Species of the genus Physa are aquatic pulmonates with simple, sinis-
trally spiral shells. They are quite sensitive to local environmental factors,
and thus recognition of biological species is extremely difficult. The result has
been proliferation of names ad nauseam. For many years F. C. Baker (1928,
p. 408) preferred to recognize the genus Physella of Haldeman (1842, Monogr.
Limnaides N. America, pt. 8, p. 14, 38) on the basis of the digitations of the
mantle, but in the last years of his life Baker abandoned this stand.

Physa gyrina hildrethiana Lea 1841

Physa hildrethiana Lea 1841, Amer. Philos. Soc. Proc . , v. 2, p. 32.

Physella gyrina hildrethiana (Lea), F. C. Baker 1928, Freshwater Moll.
Wisconsin, pt. 1, p. 453, pi. 27, fig. 36, pi. 28, figs. 2-4, 7-14.

The physid shells that occur throughout the faunal assemblages at the
Strawn and Batavia localities (135-1, 135-2, 136; fig. 3) are referred to this
name combination with some misgivings, but the shells in question closely
resemble those previously referred to as P. g. hildrethiana. One of the problems
in such an assignation is that hildrethiana is described by Baker (1928, p. 454)
and others as living in temporary ponds, where few animals ever reach maturity
before the pond dries up. The shells recovered in these studies are small as
described, but there is no evidence that the ponds were in any sense ephemeral,
since they supported throughout their history abundant populations of branchiate
mollusks that do not survive drying conditions for very long. Whatever the fact may
have been, physid shells occur in moderate numbers in every faunal assemblage
studied, and with almost no variation of shell characters. The animals must
have formed a significant and persistent element of the total molluscan fauna.

Genus Aplexa Fleming 1820

Although the shells of this genus are sinistral like those of Physa ,
animals have been separated from that genus on the basis of mantle and other

20 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

anatomical characters. The differences are not striking, and the validity of the
genus may be questioned on biological grounds. As recognized, however, the
genus is Holarctic in distribution.

Aplexa hypnorum (Linne) 1758

Bulla hypnorum Linne 1748, Syst. Nat. Ed. 10, p. 727.
Aplexa hypnorum (Linne), F. C. Baker 1928, Freshwater Moll. Wisconsin,
pt. 1, p. 473, pi. 19, figs. 1-4.

Whatever may be the true systematic relations of this species, the shells
are readily recognized by their sinistral spirals, polished surface, and narrow,
elongate spire. A. hypnorum is distributed in northern North America (La Rocque,
1968, p. 552, however, notes a record from Louisiana), northern Europe, and Asia

In our collections, A. hypnorum occurs as a single, immature shell at the
8- to 10-foot level of locality 135-1 (fig. 3). It is assumed that the species is
not a part of the indigenous faunal assemblages.

Family Carychiidae

Genus Caryohium Muller 1774

Species of Caryohium possess minute elongate spiral shells that bear a
superficial resemblance to shells of pupillid gastropods. The animals live in
damp situations, often very near water but never in it; they live as terrestrial
pulmonates. The validity of the several named "species" in North America has
been questioned with considerable reason; the problem has been ably discussed
by La Rocque (1970, p. 558).

Caryohium exile Lea 1842

Caryohium exile H. C. Lea 1842, Amer. Jour. Sci., 1st ser., v. 42,
p. 109, pi. 1, fig. 5.

Caryohium exile Lea, Pilsbry 1948, Land Moll. N. America, v. 2, pt. 2,
p. 1058, figs. 561c, 566a.

Since this gastropod is represented in the two bog collections by a single
shell (fig. 3) occurring in the base of locality 135-1, precise identification was
not possible, since shells must be dissected to demonstrate the character of the
plait that ascends the columella. In any case, the single example of Caryohium
cannot be viewed as significant in the total faunal complex.

Suborder Stylommatophora
Family Endodontidae

Genus Helioodisous Morse 1864

Gastropods of this genus have small discoid shells with tightly wound
whorls; the shell may resemble a gyraulid. In fact, Thomas Say described the
first species that subsequently was recognized as a member of the genus
Helioodisous -, the shell that he observed was discovered in a dried-up pond near
what is now Council Bluffs, Iowa, and he described it as a species of Planorbis.
However, the striking parallel striations of H. parallelus are entirely different
from any of those seen among the planorbids. The species of Helioodisous in
North America are found primarily east of the Rockies, although there are ex-
ceptions to this generalization.

POST-WOODFORDIAN MOLLUSKS 21

Helioodisous parallelus (Say) 1821

Planorbis arellelus Say 1821, Acad. Nat. Sci. Philadelphia Jour., v. 2,
p. 164 (corrected to parallelus in the index , p. 407).

Helioodisous parallelus (Say), Pilsbry 1948, Land Moll. N. America,
v. 2, pt. 2, p. 62 5, fig. 339.

This small discoid shell is represented in the present collections by a
single specimen found at the 8- to 10-foot level at locality 135-1. Although it
has been described as a bog species by Ingram (1946, Nautilus, v. 59, p. 91),
one example here among many thousands of shells does not indicate that it
flourished among the molluscan assemblages under observation.

Family Succineidae

Genus Suooinea Draparnaud 1801

The succineids are an ancient group of terrestrial gastropods with simple
shells featuring a large ultimate whorl and aperture and scarcely four whorls.
On the basis of the soft anatomy, four genera are recognized, but in general
the shells do not reflect this. Therefore, fossil shells are referred to modern
genera with varying degrees of confidence, including none at all. This report
uses Suooinea as the generic name, with the reference to other genera enclosed
in parenthesis as if they were subgenera. Considering the difficulties involved,
it would seem better, for paleontologists at least, if the modern genera were
considered subgenera of the genus Suooinea.

The succineids are worldwide in distribution, and are found on all
continents as well as on many oceanic islands.

Suooinea (Oxyloma) retusa Lea 1834

Suooinea retusa Lea 1834, Amer. Philos. Soc. Trans., v. 5, pi. 19,
fig. 86.

Oxyloma retusa (Lea), Pilsbry 1948, Land Moll. N. America, v. 2,
pt. 2, p. 785, pi. 2, figs. 25, 26.

Suooinea retusa was recovered from three faunal associations at Strawn
NE locality 135-1, and in these the species was represented by only a few
shells. These were unusually small, but they bore other characters of the
species, especially the shape of the shell, which is quite distinctive. Living
colonies of S. retusa inhabit the wet, brushy borders of the lake, especially
southwest of boring 135-1, but these modern examples have shells unusually
large for the species. The significance of these observations is not clear.
S. retusa is found principally in northern United States and southern Canada,
invariably in wet situations near streams, ponds, and lakes.

Suborder Orthurethra
Family Pupillidae

Genus Vertigo Milller 1774

The tiny shells of the pupillids are typically elongate spiral in form,
rimate, ovoid, with flaring peristome around an aperture that usually contains
a series of one to six or more denticles, although sometimes bearing none. The
family is divided into several genera, all differentiated primarily on the basis
of shell characters. The genus Vertigo is one of the most modified of the genera
The family is not only worldwide in its present distribution, but it is one of the

22 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

most ancient (geologically speaking) of the terrestrial gastropods. Typical ex-
amples of Vertigo have been recovered from continental sediments of early
Cretaceous age.

Vertigo modesta (Say) 1824

Pupa modesta Say 1824, Long's Exped., Appendix, p. 259, pi. 15, fig. 5
Vertigo modesta (Say), Pilsbry 1948, Land Moll. N. America, v. 2,
pt. 2, p. 982, figs. 528, 531.

Vertigo modesta is a species of wet situations, but it is nevertheless a
strictly terrestrial pulmonate gastropod. Empty shells of pupillids can float for
long periods of time and over long distances; therefore an occasional shell in a
water-laid deposit is not at all remarkable.

In the present study, V. modesta is represented by a single shell recov-
ered from the uppermost zone of locality 135-2. This occurrence is not consid-
ered significant in the interpretation of the total faunal assemblages.

ECOLOGICAL IMPLICATIONS OF THE BOGS AND THEIR MOLLUSCAN FAUNAS

Depositional Ecology of the Bog Sediments

It is evident from the local topography (figs. 1, 2) that each of the two
bogs under study originated in an undrained depression in morainal topography.
To what extent, if any, isolated glacial ice masses filled these depressions is
not clear. It is also unclear to what extent the original lakes in these depressions
formed from meltwater in situ and to what extent by runoff after precipitation on
the surrounding slopes, nor does it seem very important. Possibly the temper-
ature of meltwater from a slowly dissipating ice mass might have had some inhib-
iting effect upon the development of plants and animals in the associated lake,
but at the latitude of northeastern Illinois this does not seem likely, especially
under the climatic regimen that was producing ablation of the last Woodfordian
glacier in the area at the time. A sizable flora and microfauna have developed,
for example, in glacier-fed lakes in Antarctica, where climatic extremes are
certainly greater than those that could have been expected in Illinois under then
prevailing conditions.

Whatever may have been the original source of the water in these lakes,
it seems obvious that they have been maintained by runoff from the surrounding
slopes since the lakes began accumulating sediments some 10,000 or more
years ago. Furthermore, the lakes attained in former times a level distinctly
higher than the present level. Evidence for this is especially clear at the Strawn
NE locality, where a plain some 200 yards wide, north of the main body of the
lake, is judged to be a wave-cut terrace about 10 feet higher than the present
level of the water (figs. 1,2).

The deposits in the former lakes that provide the samples from which the
molluscan faunas were obtained certainly owe their origin to materials washed
from surrounding slopes; although this conclusion needs no defense in a closed
basin, the best positive evidence here is that the less than 2 micron fraction of
clay minerals is indistinguishable from that obtained from the tills of the area
(H. D. Glass, 1972, personal communication) .

POST-WOODFORDIAN MOLLUSKS 23

In each case, the drainage area supplying runoff to the bog lakes is
small; Strawn NE (fig. 1) has an effective drainage area (bounded by the 750-
foot contour) of less than a square mile. At Batavia W (fig. 2) the drainage area
is somewhat larger (bounded approximately by the 720-foot contour) and en-
compasses well over a square mile.

The rate of sedimentation into the bog lakes has been slow. Assuming a
conservative figure of 10,000 radiocarbon years B.P. as the age of the deposits
and 200 inches as the depth of the sediments, the average rate of accumulation
has been 0.02 inches per year, or 2 . 0 inches per century. In spite of the small
size of the drainage areas of the two bogs, it appears that: (1) rainfall during
the post-Woodfordian interval must have been minimal, and/or (2) the slopes
must have been well protected from erosion by a dense vegetative cover.

At the Strawn NE bog lake, the sediments are extremely fine grained,
and pebbles are rare except near the base of the lake deposits. The borings
might well have missed any gravel trains from entering drainage, but another
observation is more difficult to dispose of. For many years, the marl deposits
at Strawn NE were removed by a jury-rigged dragline that stretched for a
long way across the lake and was situated near the entrance into the lake of a
drainageway from the east (fig. 1) . Where the dragline scoop was emptied into
trucks, a large amount of marl collected, and since abandonment of the har-
vesting operation several years ago, this "spoil pile" has been subject to wind
and rain. Nevertheless, no pebbles were to be found, although the accumulation
must represent a rather large sample area. The conclusion seems inescapable
either that runoff into the bog lake was not competent to move rock particles
larger than silt and very fine sand, or that larger particles were not available
because of the protection of covering vegetation on the surrounding slopes.
Heavy vegetative cover should result in populations of terrestrial mollusks, but
there is almost no evidence of these animals, from the borings, from the large
numbers of fossil shells exposed by recent wave action, or from the hundreds of
shells exposed on the spoil pile previously referred to.

The sediments contain fossil plants (diatoms) in the basal sediments;
in the upper half of the deposits, plant remains become very conspicuous, but
are decomposed beyond the possibility of identification. A few fossil seeds
were recovered, most of them from sedges {Cyperus sp.). Pollen was not inves-
tigated; but since the bogs are alkaline, pollen may be poorly preserved.

Each of the two bog lakes terminated in a matte of sphagnum peat, not
more than 12 to 18 inches thick at Strawn NE but 4 to 5 feet thick at Batavia
W. The base of this peat is approximately 2,000 radiocarbon years old
(ISGS-134).

Ecological Implications of the Molluscan Faunas

The molluscan faunal assemblages recovered in the two bog deposits
are predominantly aquatic in habitat requirements; of the six taxa that occur at
every level in both bogs (fig. 3), all are aquatic and three are branchiate
gastropods. Ten species occur in more than 20 of the 31 faunal assemblages;
of these, five are obligate aquatic animals {Amnicola gelida, A. leightoni,
Valvata trioarinata, Pisidiwn nitidum, and P. eompressum) using gills for gas-
eous exchange, although they may be able to exist for brief periods of time
without open water. Terrestrial gastropods are very rare in these collections;

24 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

among the four species that may be characterized as terrestrial {Suoainea retusa,
Caryehium exile, Helicodiscus parallelus > and Vertigo modesta) , only S. retusa
is represented in more than one faunal assemblage, and by more than one shell.
In short, the molluscan species in the assemblages under study here are over-
whelmingly aquatic in habitat requirements.

Unfortunately, little is known specifically about the habitat requirements
of freshwater mollusks . As long ago as 1932 Morrison studied the molluscan
faunas in the Northeastern Wisconsin Lake District and noted the hydrogen ion
concentration (pH) and dissolved carbonate in the waters of the various lakes;
the latter datum he reported in parts per million. Subsequently Morrison has
been widely quoted on the assumption that his observations establish the limits,
in terms of pH and dissolved carbonates, of the molluscan species reported.
In point of fact, his observations can be considered no more than what they
were — observations. It is probable that the lakes do extend beyond the limits
of tolerance as far as acidity is concerned, but no limits were even hinted at on
the alkaline end of the scale, and no laboratory experiments were carried out to
determine tolerance limits. Mollusks which have shells require carbonate for
their development; these animals obtain carbonate directly from ambient waters
or from plants which they consume. The range of pH in which freshwater mollusks
may live and reproduce is extremely great; Hunter (1964, p. 87) reports that four
species of freshwater pulmonates that transmit schistosomes were shown by
laboratory experiments to be able to breed in waters varying from pH 4.8 to pH
9.8. Hunter found (1964, p. 86) that while pH may be a limiting factor in the
distribution of freshwater mollusks, "... the most important chemical factor
is dissolved calcium, varying more than a hundred-fold in fresh waters with
mollusks. "

Morrison points out that, normally, thin-shelled unionid mussels in
waters with low pH and low "fixed carbon dioxide" have shells so flexible that
they may be distorted by twisting without breaking (1932). Waters with less
than 3 mg/liter of calcium support very few mollusks; in Britain such waters
were observed to support a species of Lymnaea, one of Ancylis (Ferrissia) ,
and several species of Pisidiwn. Waters having 8 to 10 mg/liter of calcium
support up to 17 molluscan species, while an Irish lake having about 50 mg/liter
of calcium supports 32 species (Hunter, 1964, p. 86). Boycott (1936) reports
that of 62 species of freshwater mollusks in Britain, 26 can live in waters con-
taining less than 10 mg/liter of calcium and 6 occupy lakes and streams having
more than 10 mg but less than 20 mg/liter, while the remainder required waters
having at least 20 mg/liter of calcium.

Temperature is not likely to have been a problem at the two bog lakes
under investigation, as it is well known that mollusks can survive wide ranges
of temperature. Odhner (1923) in a report on Novaya Zemlya, an island off
northeastern Siberia well above 70° N latitude, found Pisidiwn oonventus active
in the brief summers, where winter temperatures are extremely severe. Lymnaea
peregralives in thermal waters at 45° C in the Pyrenees, while the same species
has been observed active under ice in lakes in Scotland (Hunter, 1964, p. 86).
Even from these brief observations and from the modern distribution in North
America of the species listed in figure 3, it can be concluded that temperature
could not have been an important factor in the survival of the species recovered
from the two bogs.

From available evidence, it is probable that the trophic state of the two
bog lakes was favorable to the growth of mollusks; the accumulation of organic

POST-WOODFORDIAN MOLLUSKS 25

materials, including diatoms in the lowermost sediments, and increasing quan-
tities of plant remains as the lakes increased in age, point to a condition of
eutrophy. " Mollusks are most plentiful in eutrophic lakes with hard water, less
common in oligotrophic lakes, and absent from certain dystrophic lakes with
little calcium" (Hunter, 1964, p. 87). Inasmuch as the two bog lakes were
apparently eutrophic in a strongly basic environment since marl formed in large
quantitites, it is not surprising that populations of many species of mollusks
were maintained there for a long period of time (approximately 10,000 radio-
carbon years) .

Several studies of bog lakes or ancient bog deposits have been made in
the general latitude of northeastern Illinois, one of the most interesting and
informative of these being an investigation of Dollar Lake, a few miles northeast
of Kent, Portage County, Ohio (Dexter, 1950). Dexter attempted to correlate
molluscan faunal assemblages with vascular plant zones and with pH. He found
that at depths greater than 12 feet, the bottom was bare of vascular plants and
devoid of mollusks. He also found that in spite of the general basic condition
of a bog, surface waters reached a pH of 6 . 0 at the time of the spring turnover.
Unfortunately, Dexter makes no mention of precise quantities of dissolved car-
bonates in the water. He reported the occurrence of 12 aquatic mollusks, 5 of
them common to the Strawn-Batavia faunas. These are: Promenetus (Menetus)
exaouous , Valvata tricarinata, Eelisoma oampanutatum , Lymnaea obrussa, and
L. palustris. He also mentions Suooinea retusa, and several additional aquatics
by genus only.

Reynolds (1959) described the molluscan fauna in a covered bog deposit
from a cornfield in Ohio; the recovered faunal assemblages were entirely aquatic.
He analyzes the various species as to percent of the total, but does not specify
whether the figures were arrived at from population numbers or by some other
means. La Rocque (1952) in an earlier study speaks of percent "by volume."
Reynolds uses Morrison's data from a study of molluscan faunas in Wisconsin
lakes (without reference), but strangely speaks of "fixed carbon dioxide ratio"
in parts per million. Reynolds accepts Morrison's data as if they established
the limits of tolerance of the various species concerned. Reynolds discusses
the absence of terrestrial elements in the faunas and concludes that terrestrial gas-
tropod shells could not drift as far as 150 feet, the approximate distance of the exca-
vation from the original soil. Reynolds reports the following species, which are com-
mon to the Strawn-Batavia faunal assemblages: Pisidium compression, P. nitidum,
Sphaerium sulcatum (= S. simile!) , Amnicola leightoni, Lymnaea obrussa,
Gyraulus altissimus , Eelisoma anceps (= antrosa) , Eelisoma trivolvis , Physa
gyrina, Promenetus exaouous , and Valvata trioarinata. In all, Reynolds reported
7 species of sphaeriids and 11 kinds of gastropods.

Another significant study of ancient lake faunas was made by Zimmerman
(1960), who described 38 species of mollusks taken from the Newell Lake de-
posit in Logan County, Ohio. Among the 21 kinds of aquatic mollusca, 13 are
common to the Strawn-Batavia assemblages: Valvata lewisi, Amnicola leightoni,
Pseudosuccinea columella, Lymnaea obrussa, Eelisoma antrosa, E. campanulatum ,
E. trivolvis, Planorbula armigera, Promenetus exaouous, Gyraulus altissimus,
Ferrissia parallela, and Physa gyrina. He also noted Succinea retusa. Zim-
merman also utilizes the data of Morrison on pH and "fixed carbon dioxide" as
if they established tolerance limits for various species. He compares his
faunas with Pleistocene molluscan faunas reported by Baker (1918).

26 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

Baker (1918, p. 659) lists mollusks recovered from four ponded deposits
on the Champaign Moraine in Urbana and nearby Mahomet. Recognizable species
comparable to the Strawn-Batavia assemblages include: Valvata tricarinata,
Sphaerium sulcatum (= S. simile1?), Pisidium compression, Helisoma antrosa, H.
campanulatum , H. trivolvis , Gyraulus altissimus , Physa gyrina, Lymnaea
caperata, and L. obrussa. In a later study, Baker (1930) reported these species,
which are common to the two bog assemblages: Sphaeriwn sulcatum ( = S. similel) ,
Valvata tricarinata , Helisoma antrosa, and Ferrissia parallela.

More recent studies of fossil molluscan assemblages include those of
Leonard and Frye (1960), who reported on Wisconsinan molluscan faunal assem-
blages in the Illinois Valley region. Species common to the Strawn-Batavia
faunal assemblages include: Carychium exile, Helisoma trivolvis, H. antrosa,
Planorbula armigera, Gyraulus altissimus , Lymnaea dalli, L. parva, L. palustris ,
Valvata tricarinata , and Amnicola leightoni. Lymnaea dalli and L. parva were
frequently recovered from loess deposits; the animals apparently lived near small
ephemeral ponds on the loess surface, although other evidence of these ponds
was not always apparent.

In a report on Illinoian and Kansan molluscan faunas, Leonard, Frye, and
Johnson (1971) listed 65 species of aquatic and terrestrial mollusks from 29 lo-
calities; among those species common to the Strawn-Batavia faunal assemblages
are: Aplexa hypnorum, Armiger exigua, Gyraulus altissimus , L. dalli, L. obrussa,
L. palustris, Pisidium compressum, Valvata tricarinata , and Vertigo modesta.

A study of the geology and paleontology of Pleistocene Lake Saline in
southeastern Illinois (Frye, Leonard, Willman, and Glass, 1972) revealed several
species common to the Strawn-Batavia assemblages: Promenetus exacuous ,
Amnicola gelida, A. leightoni, Gyraulus altissimus, and Valvata tricarinata.

One of the conclusions to be drawn from these observations on previous
occurrences is that the faunal assemblages found at the Strawn and Batavia
localities, in spite of being relatively young, are composed of molluscan species
established in Illinois since much earlier Pleistocene times. Most of these
species also occur in Pleistocene faunal assemblages in other areas, as, for
example, in the studies in Ohio previously referred to.

Inasmuch as it has been shown that freshwater mollusks are capable of
living in waters that vary greatly as to temperature, pH, dissolved minerals,
and trophic state, the ecological inferences which can be drawn from the mollus-
can fossils recovered from the two bogs in question are somewhat limited.

1 . An unexpected result of an analysis of the molluscan assemblages is
that the molluscan faunas in the two bogs are strikingly similar; such differences
as do occur are apparently not important. For example, Helisoma campanulatum
is found at the Strawn NE locality in only 2 of 20 collections, while at Batavia
W the species occurs at every level investigated (fig. 3). The significance
of this difference in occurrence is not clear.

2 . At neither of the two bogs, at any time in their history extending
over 10,000 radiocarbon years or more, were freshwater molluscan faunas as
abundant as the number of fossil shells per cubic unit would indicate, because
the rate of deposition was extremely slow (2 inches per century) . Since no
etched shells were observed at any level, it can be concluded that most of the
shells that settled to the bottom were preserved. The presence of intact pairs
of sphaeriid shells bolsters this conclusion. It therefore follows that molluscan

POST-WOODFORDIAN MOLLUSKS 27

populations were not abundant at any time in the history of the Strawn NE and
Batavia W bogs, since population density per cubic unit is essentially constant
except in the lowermost few inches of the deposits.

3 . Although there is evidence of higher water levels in the basins that
now contain the bogs and bog lakes at Strawn NE and at Batavia W, there is
also some evidence that water levels fluctuated widely. Physa gyrina
hildrethiana is recognized as especially adapted to surviving ephemeral pools,
and as a result seldom reaches full size. The almost uniformly small size of the
examples recovered tends to support this thesis, although obligate aquatic mol-
lusks occur in the same samples. The conclusion is that P. g. hildrethiana
lived in peripheral parts of the lake, and were left stranded as water levels fell,
while amnicolids and other branchiates lived in the open waters at the center of
the bog basin.

4. The sparsity of specific data concerning the ecological requirements
of freshwater mollusks makes it difficult to describe the conditions under which
the mollusks lived in the Strawn NE and Batavia W bogs. The most that can be
done is to use the time-honored (but not necessarily accurate) system of com-
paring the fossil assemblages with living molluscan faunas. On this basis, it
can be concluded that the molluscan faunal assemblages in the two bog lakes
lived in relatively clear, cool waters, moderately eutrophic, basic in reaction,
sufficiently supplied with dissolved carbonates to support strong growth of mol-
luscan shells, and characterized by fluctuating levels. This favorable environ-
ment drew to a close about 2,000 radiocarbon years B.P. when, apparently
because of declining water levels, sphagnum moss produced a matte that prob-
ably covered the entire surface of the open water. Molluscan populations
declined sharply at this time; shells are found only in the lowermost 2 to 3 inches
of the peat. Molluscan populations in the two lakes are now very sparse.

5. No evidence of the Altithermal Interval at about 7,500 radiocarbon
years B.P., which should occur at approximately the 10-foot level in the two
bogs, can be seen in the faunal assemblages.

Finally, as far as is known to me, the molluscan faunal assemblages
reported here are the first freshwater faunas reported from the time interval
10,000 to 2,000 radiocarbon years B.P. and documented by direct radiocarbon
analyses.

Core samples would add to the accuracy of the knowledge of the vertical
distribution of mollusks in these sediments; and studies of diatoms, ostracods,
and pollen would broaden the scope of an understanding of the post-Woodfordian
geological history.

REFERENCES

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mollusca: Bull. 111. State Lab. Nat. Hist., v. 8, p. 441-499.
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Sci., Spec. Publ. No. 3, p. i-xvi, 1-539, 58 pis., 51 figs, in text.
Baker, F. C, 1918, Post-glacial mollusca from the marls of central Illinois: Jour. Geology,

v. 26, p. 659-671.
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28 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487

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by Harley Jones Van Cleave): Urbana: Univ. 111., xxxiv+53 0 p., 14 1 figs.
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(General), 112 p., 2 figs.
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(Pelecypoda), p. 113-356, 8 pis., 206 figs.
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(aquatic Gastropoda), p. 357-554, 6 pis., 200 figs.
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(terrestrial Gastropoda), p. 554-800, 4 pis., 175 figs., index.
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Illinois State Geological Survey Circular 487

28 p., 3 figs., 2800 cop. , 1974

Urbana, IL 61801

Printed by Authority of State of Illinois, Ch. 127, IRS, Par. 58.25

CIRCULAR 487

ILLINOIS STATE GEOLOGICAL SURVEY

URBANA, IL 61801