48 THE 
CHOLOGISTS’ 
EXCHANGE. 
BRIEF NOTES ON THE LAND AND 
FRESH-WATER SHELLS OF MER- 
CER CO., ILL. 
BY WILLIAM A. MARSH. 
: (Continued. ) 
FAMILY CORBICULAD-. 
SPHASRIUM AND PISIDIUM. 
The family Corbiculadze is divided into six 
genera, as follows: 1 Corbicula, Megerle; 2 
Batissa, Gray; 3 Velorita, Gray; 4 Cyrena, | 
Lamarck; 5 Sphzerium, Spe 6 Pisidium, 
Pfeiffer. Bafiess and Vv elorita are not repre- 
sented on this Continent. Only two genera 
are represented in Mercer County, viz: Sphve- 
rium and Pisidium, seven species being found 
of the former and three of. the latter. The 
animal of Sphzerium is oval, lobes of the mantle 
simple, united pesteriorly, and terminating in 
two syphons joined at the base without tentacles, 
“mouth oval, small tentacles of the mouth short 
and narrow. Gills rather broad, nearly equal, 
united behind the foot, which is narrow and 
elongated. The shell of Sphzerium is oval, 
nearly equilateral, beaks somewhat prominent, 
hinge margin narrow, with two primary teeth 
in each valve, lateral teeth elongated, pallial 
impression simple, ligament external, narrow, 
and situated on the longer portion of the shell. 
‘The species composing the genera, Spheerium 
and Pisidium are small bivalves found: in all 
our creeks and in the Mississippi River, also 
in the numerous sloughs and small lakes of our 
county. Like our Uniones, some species are 
more active than others; some are abundant, 
while others are rare 
58.—Spherium sulcatun, Lamarck. 
‘This fine species, as found here, varies some- 
what from typical forms. It is usually trans- 
versely oval, nearly equilateral, rounded ante- 
riorly, pointed posteriorly, slightly curved at 
base, valves convex, beaks full, and raised 
above the outline of the shell, with coarse and 
regnlar sulcations. 
double in both valves. I have found this spe- 
cies, which is rare here, in two or three locali- 
ties in Pope and Edwards Creeks, and at the 
mouth of Edwards Creek in the Mississippi 
River, usually associated with s¢¢afinem and 
(Vr ANSUVCVSUM. 
59:-—Spherium solidulum, Prime. 
Shell transversely inequilateral, 
sometimes slightly convex, sometimes com- 
pressed, beaks full, not prominent, anterior 
margin rounded, slightly angular posteriorly. 
Epidermis variable from dark brown to brown- 
ish yellow, sometimes greenish, interior dark 
_ blue, cardinal teeth double, lateral teeth large. 
This species somewhat resembles sz/ca¢uz, but 
is less elongated, less convex, more solid, with the 
sulcations rather coarser and more irregular. 
This shell is very rare here, as I have found it 
in but one locality, and that in Edwards Creek. 
elongated, 
60.—Spherium striatinum, Lamarck, 
Shell rather thin, transversely elongated, 
sometimes inflated and somewhat compressed, 
inequilateral, rather rounded anteriorly, distend- 
ed posteriorly ; beaks full but not much raised, 
suleations variable, sometimes coarse, at times 
nearly smooth. * Epidermis greenish yellow 
to light brown, shell blue within; cardinal teeth 
double and very small. his well-known spe- 
cies is found rather commonly 1 in our creeks as 
well as in the river, and its sloughs and lakes. 
It is our most common Sphertum, and 1 fnd 
it very difficult to Separate from .S. so/idudume. 
It is found here hoth in deep and shallow 
water, in both swift and quiet water, and in 
the mud as well as in the sand and gravel. 
67.—Spherium fayanum, Prime. 
a 2, ’ 
Shell rhombic, nearly equilateral, convex, 
thin, anterior and posterior margins very abrupt, 
beaks central, calyculate, approximate at apex. 
Cardinal teeth distinct; lateral teeth “long; 
shell blue within, strize hardly visible. Epider- 
mis light greenish horn color, often having a 
bright yellow band on the inferior margin. 
A rare and beautiful species, found only in 
Epidermis dark chestnut | the river and its sloughs, and the small lakes 
brown, interior light blue, cardinal teeth small, _ of the Bay Island. 
It, in some respects, favors 
