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MOLLUSCS, VERMES AND HYDROZOA 
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furnishes lime for the shells of the snails without depriving other animal 
or plant life in the aquarium thereof. All water under natural conditions 
contain mineral salts, but those of the aquarium may become exhausted, 
and it is requisite that they should be replaced. 
CLASSIFICATION OF THE Bivatves. Freshwater mussels belong to 
the order of the Lamellibranchiata and are divided into two families, the 
Cycladide and the Unionide. The following classification will briefly 
describe the order, families, genera and species, and serve asa key to the 
subsequent descriptions, which are confined to Eastern and Middle 
States mussels, though many are common to other sections of the 
United States. 
OrpER J[AMELLIBRANCHIATA. 
Family Cycladida—Valves small, suborbicular, hinge with cardinal and lateral teeth, 
animal with open simple mantle, siphon more or 
less united, two unequal gills each side, foot large 
and tongue-shaped. 
Genus Spherium— 8. simile, S. striatinum. 
Genus Pisidium—P. compressum, P. abditum. 
Family Unionide—Valve large inequipartite oval or elongated, hinge with a simple 
or divided cardinal tooth in each valve and an 
elongated lateral tooth, gills free from the abdom- 
inal sac with dorsal attached to mantle, upper 
siphonal opening somewhat fringed. 
Genus Unio—U. comp lanatus. 
Genus Lampsilis—L. radiosus, L. ochraceus, L. cariosus. 
Genus Anodonta—J. cataracta, A. implicata. 
Genus Margaritana—M. margaritifera, M. marginata. 
SpH@#rium. These small mussels are generally distributed and have 
thin, ovate-globose shells, and the hinge has two minute cardinal teeth in 
each valve, sometimes but one, and compressed marginal teeth. They 
are seldom over 34 inch long. 
S. simile, Fig. 180, is the larger species of the Eastern Section and 
has sub-oval dark chestnut-brown very convex valves, varying considera- 
bly in outline. In the adult the extremities are broader and nearly equally 
rounded, the posterior part somewhat longer 
and more pointed and the umbones nearly 
central; while with the young the light-yellow 
valves are thin, compressed and the hinge 
margins nearly a straight line. ‘The surface 

of the adult shell is concentrically wrinkled with = SS 7ZZ- 
distinct lines of growth, and the hinge has FIG. 180. Spharium simile. 
Enlarged. 

oblique minute cardinal teeth, and those of the 
margin are distinct, strong and white. The valves are 34 inch long, % inch 
broad and 2 inch thick. The animal is of a light salmon-pink color and 
240 
