Threeridge
Amblema plicata (Say, 1817) |
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Threeridge |
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| Amblema plicata, INHS 9062. Spoon River, Fulton County, Illinois. Length: 4.2 inches (10.7 cm).
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| Other common names Blue-point, purple-tip, fluter.
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| Key characters Elongate or rounded shell with ridges or folds on the posterior half. No sculpturing on the anterior end, especially near the umbo. | |
| Similar species Washboard, rock-pocketbook.
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| Description Shell thick, elongate or rounded, and compressed to
inflated. Anterior end rounded, posterior end rounded or bluntly pointed.
Dorsal margin straight, ventral margin straight to curved. Umbos low, equal to
or slightly elevated above the hinge line. Beak sculpture of three or four
ridges usually seen only in young shells. Shell smooth on the anterior third,
with three or more roughly parallel ridges or folds on the posterior
two-thirds. Shells from large rivers generally more inflated and without as
many folds or wrinkles as those found in smaller streams. Periostracum
yellowish green, light brown, or green in juveniles, becoming dark green,
brown, or black in older shells. Length to 7 inches (17.8 cm). Pseudocardinal teeth serrated and well developed; two in the left valve, one in the right (occasionally with a small tooth on either side). Lateral teeth serrated and straight to slightly curved; two in the left valve, one in the right. Beak cavity medium to deep. Nacre pearly white, frequently stained, iridescent posteriorly. Some individuals with a purple tint on the posterior end.
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| Habitat Small to large rivers and impoundments in mud, sand, or
gravel.
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Status Widespread and common throughout most of its range. Apparently
declining in some smaller streams. Commercial species.
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