Incongruous Ark
Scapharca brasiliana (Lamarck, 1819)
Identified by beaded ribs extending from umbo. The Incongruous ark is a fairly common shell on S. Hutchinson Island beaches.
Incongruous Ark, S. Hutchinson Island 2020
Inside view:
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Incongruous ark
Scapharca brasiliana
Family Arcidae
Distinguishing Characteristics
Bivalve (2 shells); shell ovoid, slightly triangular in shape, anterior end bluntly rounded, posterior end more angular; beaks large, separated (not touching) when view from the side, hinge line (edges between beaks) straight on top, slightly curved below; ligature area beneath beak (externally) triangular shaped; shell color white, brown furry covering (periostracum) when alive; heavily beaded radiating ribs on left valve, right valve has smooth radiating ribs, beaded only on anterior and posterior ends; beads somewhat elongate (wider than tall); smooth radiating ribs on internal surface; edges of shell crenate (scalloped); left valve overlaps right valve; comb-like teeth on interior of hinge line, teeth becoming smaller towards the center; ventral edge of left shell curved slightly outward when viewed from the ventral side.
Similar Species
The incongruous ark is most similar to the triangular ark. The triangular ark has beads that are not as oblong (except possibly on sides) and the shell has a straight ventral anterior edge.
Habitat
Gulf and bay, sandy bottoms
Maximum Size
6.2 cm (2 1/2 in)
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Incongruous Ark:
Plentiful and Beautiful
By Patricia B. Mitchell.
The well-inflated (highly-rounded) Incongruous Ark is unusual in that the bivalve's basal ventral margin, or edge, curves gently instead of forming a straight line. This curve can be seen by holding the complete bivalve, both halves joined, with the beaks facing away from yourself. You can see a slight curve on the bottom edge.
The two-inch white shell can be found on beaches from North Carolina to Brazil. The exterior is scored with broad, beaded ribs. The beak is centered with the smaller hinge teeth closest to it, and the larger hinge teeth out farthest on the hinge line.
Though this is a commonly-found shell, one should not ignore its beauty.
(The photo shows two white Incongruous Ark halves and one discolored valve. Note especially the beading on the darker shell.)
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Bivalve (2 shells); shell ovoid, slightly triangular in shape, anterior end bluntly rounded, posterior end more angular; beaks large, separated (not touching) when view from the side, hinge line (edges between beaks) straight on top, slightly curved below; ligature area beneath beak (externally) triangular shaped; shell color white, brown furry covering (periostracum) when alive; heavily beaded radiating ribs on left valve, right valve has smooth radiating ribs, beaded only on anterior and posterior ends; beads somewhat elongate (wider than tall); smooth radiating ribs on internal surface; edges of shell crenate (scalloped); left valve overlaps right valve; comb-like teeth on interior of hinge line, teeth becoming smaller towards the center; ventral edge of left shell curved slightly outward when viewed from the ventral side.
Similar Species
The incongruous ark is most similar to the triangular ark. The triangular ark has beads that are not as oblong (except possibly on sides) and the shell has a straight ventral anterior edge.