MOLLUSCS, VERMES AND HYDROZOA 



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 Bivalves. Freshwater mussels belong to 

 the order of the Lamellibranchiata and are divided into two families, the 

 Cycladidse and the Unionidae. The following classification will briefly 

 describe the order, families, genera and species, and serve as a 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. 



Order Lamellibranchiata. 



Family Cyckdidas — 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 Sphaerium — S. simile, S. striatinum. 

 Genus Pisidium — P. compressum, P. abditum. 

 Family Unionids — 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. comflanatus. 



Genus Lampsilis — L. radiosus, L. ochraceus, L. cariosus. 

 Genus Anodonta — A. cataracta, A. implicata. 

 Genus Margaritana — M. margaritifera, M. marginal a. 



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 ^ inch long. 



S. simile^ Fig. 1 80, 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 

 distinct lines of growth, and the hinge has 

 oblique minute cardinal teeth, and those of the 



margin are distinct, strong and white. The valves are ^ inch long, ^ inch 



The animal is of a light salmon-pink color and 



FIG. 180. Sphterium simile. 

 Enlarged. 



broad and | inch thick 



240 



