Strophomena concordensis

• Typically larger than other forms of Strophomena
• Subtriangular shape of shell
• Posterior of the dorsal (brachial) valve is distinctly flattened
• Delicate striations, no fold or sulcus

• Articulate brachiopod. Differs from similar species by its large size, triangular outline, delicate striations, absence of sulcus and fold, absence of conspicuous thickening of anterior margin, and muscle area on inside of pedicle valve not strongly fan-shaped nor ornamented with radial ridges.

Foerste (1912):

• Strophomena concordensis evidently is closely related to Strophomena nutans, but it is a much larger species than the typical form of the latter. Specimens frequently attain a width of 42 mm., a length of 30 mm., and a convexity varying from 11 to 15 mm. The convexity of the more strongly arched specimens varies between 0.44 and 0.48, although frequently less. The subtriangular appearance of the brachial valve is strengthened by the downward deflection of the antero-lateral parts of the valve. The posterior part, within 10 mm. of the hinge-line, usually is distinctly flattened, but the concavity of the valve immediately anterior to the beak is almost imperceptible. The convexity of the umbonal parts of the pedicel valve, immediately anterior to the beak, is moderate, but is much more readily perceptible than the corresponding almost imperceptible umbonal concavity of the brachial valve. The concavity of the middle part of the pedicel valve varies, of course, with the general convexity of the brachial valve. In extreme cases it is deeply concave, the maximum concavity being at or immediately anterior to the middle of the shell. Oblique wrinkling of the shell along the hinge-line may be detected occasionally. The radiating striae of the brachial valve are very fine near the beak, but become coarser toward the anterior margin of the shell.

Foerste (1908):

• This species belongs to the same group as Strophomena nutans, but it is larger than typical specimens of that species, and the interior of the pedicel valve is never thickened as strongly or abruptly as in that form. The thickening of the interior of the pedicel valve, in fact, usually is only moderate, and is crossed by vascular markings which are more conspicuously parallel or moderately radiating along the anterior border than in any other species. The brachial valve usually is more or less nasute anteriorly, and the view of the shell from the side of the pedicel valve is more or less triangular. The convexity of the brachial valve varies considerably, but usually is rather strong. The flattened part of the valve extends only from 6 to 9 millimeters from the beak, but the rapid downward part of the curvature often does not begin until 15 millimeters from the hinge line. The radiating striations are of the type found in Strophomena planumbona. The types are from Concord, Ky.

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