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Unio pictorum (Linnaeus).
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Section of shell of Unio (A). Horizontal shell of Pinna, showing the hexagonal prisms (B)
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Typical Unio of the Mississippi Valley type, showing exterior of right valve... This is a full-grown shell, but not an old one, as the dark epidermis is but little eroded at the beaks.
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Shells of Fresh-Water Mussel (Unio crassidens). Mississippi River.
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Unio gibbosus Barnes. A species in which the male and female shells are alike. This has young filling the outer gills only.
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Unio litoralis (Cuvier).
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River-Mussel (Unio margaritifer) with pearl on one valve.
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Unio ventricosus Barnes. Showing rest periods. This specimen was collected in its eighth year..