IOWA ACADEMY OP SCIENCES. 
181 
My thanks are due to Mr. C. P. Sigerfoos for the loan of 
series of sections of two undetermined species of Sphaerium, 
with which some comparisons were made. 
EXPLANATION OP PLATES. 
a a. 
Anterior adductor muscle. 
a o. 
Anterior aorta. 
a rp. 
Anterior retractor pedis muscle. 
a s 
Anterior adductor muscle scar. 
a u. 
Auricle. 
h. 
Byssal gland rudiment. 
bis. 
Blood space. 
b s. 
Branchial siphon. 
c. 
Cloacal chamber. 
Cerebral ganglion. 
c r. 
Chitinous rods. 
c s. 
Cloacal siphon. 
f. 
Foot. 
fil. 
Gill filament. 
ifj- 
Inter-filamentar junctions. 
Inner gill. 
i o. 
Inhalent ostea. 
1. 
Liver. 
Ip. 
Labial palpus. 
m. 
Mantle. 
o B. 
Organ of Bojanus. 
ce. 
CEsophagus. 
og- 
Outer gill. 
o t. 
Otocyst. 
O V. 
Ovarian follicle. 
p. 
Pericardial cavity. 
pa. 
Posterior adductor muscle. 
Pg- 
Pedal ganglion. 
P ^P- 
Posterior retractor pedis muscle. 
ps. 
Posterior adductor muscle scar. 
psg. 
Parieto-splanchnic ganglion. 
r. 
Mantle ridge. 
r o. 
Reproductive organs. 
t. 
Male follicle. 
V t. 
Ventricle. 
w t. 
Water-tube. 
PLATE I. 
Fig. 1. A reconstruction of an adult specimen from serial sections, seen 
from the left side. Median, and the paired organs of one side 
shown. The liver and reproductive organs of older specimens 
are more extensive. 
