174 
IOWA ACADEMY OP SCIENCES. 
the meantime it seems to me that the anatomy of a single genns 
and a single species of that genus may not be wholly without 
interest, especially to those who are working in the interior, 
where the Unionidse and Cyrenidm are the only available Lam- 
ellibranchs. 
Regarding the systematic position of Sphserium, suffice it to 
say that the old genus Cyclas includes the present genera 
Sphaerium and Pisidium, and that these, with four or more 
other generally accepted genera, go to form the family which 
has been variously known as Cycladae, Corbiculidas and 
Cyrenidae. 
SHELL. 
(Pig. 2.) The shell of this species is comparatively thick, of 
a dark horn color, frequently lighter near the margins of the 
valves, and is composed of a rather thick bluish- white nacre, 
covered exteriorly by epidermis. The lines of growth are well 
marked. The teeth are thin lamellae, 2-2 on the right valve 
and 1-1 on the left valve. The adductor scars, a s and p s, are 
quite distinct and are joined dorsally by the retractor pedis 
scars. The pallial line is rather obscure. A large specimen 
measures 15x12x9 mm. 
MANTLE. 
The mantle consists of two thin lobes of connective tissue 
covered by epithelium, free at their anterior and ventral mar- 
gins, united to form the siphons posteriorly, and continuous 
over the back. The lobes lie closely applied to the shell nacre, 
which is secreted by them, and are attached to the nacre at the 
pallial line by the pallial muscles, and to the epidermis through 
the epidermal gland, which lies in a groove in the mantle mar- 
gin. A ridge. Pig. 3, r, extending from the ventral end of the 
anterior adductor muscle to the branchial siphon, runs along the 
inside of each mantle lobe near its ventral margin and serves, by 
meeting its fellow on the opposite lobe, or sides of the foot in 
case that organ is protruded, to close the open side of the branch- 
ial chamber and force currents of water to enter through the 
branchial siphon, which is protruded above the mud or sand in 
which the animal lives. The siphons. Pigs. 1 and 3, 6 s and 
c s, are quite muscular and are capable of considerable protru- 
sion. Neither one is fringed with tentacles. 
MUSCULAR SYSTEM. 
The muscular system may for convenience be classed as 
adductors, retractors, foot muscles and mantle muscles, includ- 
ing those of the siphons. 
