Species: Strophomena planoconvexa (Hall, 1847)
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Map point data provided by iDigBio.
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- C2 Sequence (Fairview: Fairmount)
Identification in Hand Sample
- Brachial valve convex to the exterior
- Pedicle valve concave to the exterior
- Convexity of the brachial valve commonly not more than 6 mm., concavity of pedicle valve 1-2 mm.
- Semi-elliptical, plano-convex shell with straight hinge line
- Radiating striae are much coarser than in most species of Strophomena
Strophomena planoconvexa from the Waynesville Formation of Brookville, Indiana (OUIP 1595)
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- Articulate brachiopod. This genus differs from Rafinesquina, by having the brachial valve convex to the exterior, and the pedicle valve concave to the exterior. Note characteristic muscle scar, also the irregular coarse striations. Fairmount. (S. planoconvexa has been tentatively regerred to Trigrammaria by some workers)
- Shell semi-elliptical, brachial (dorsal) valve moderately convex, pedicle (ventral) valve but slightly concave giving a plano-convex shell. Striae coarse, 3-4 in 2 mm. near the anterior margin. Width 25-40 mm., length, 0.7 width. Convexity of brachial (dorsal) valve commonly not more than 6 mm., concavity of pedicle (ventral) valve 1-2 mm. Foerste has suggested that the difference in form between S. maysvillensis and S. planoconvexa is one of environment. S. maysvillensis being merely the more healthy form.
- Throughout Northern Kentucky and adjoining parts of Ohio and Indiana S. planoconvexa characterizes a zone in the basal Fairmount. Farther south it occurs higher and ranges to the top of the Fairmount.
- This species is remarkable for the slight concavity of the pedicel valve. It is evident that the valve in its earlier stages of growth was slightly convex, especially near the beak, and had attained a length of 11 to 14 mm. before reversing its curvature. This concavity frequently does not exceed 1 mm. in specimens 35 mm. wide. The brachial valve usually is only moderately convex. This convexity frequently does not exceed 6 mm. in specimens 33 mm. wide, but may equal 9 mm. in the same width. The ratio of the length of the shell to the width usually varies between 0.70 and 0.80. The muscular area of the pedicel valve is shallow, but the curved lateral border is distinct, especially along the exterior edge. Anteriorly, this lateral border is deflected slightly forward, producing a median gap. The median ridge traversing the muscular area is low and rather narrow anteriorly. There are only faint traces of median vascular marking or of any thickening of the shell toward the anterior margin. The interior surface, in general, is smooth. In addition to the cardinal process, the crural plates, and the low median elevation, the interior of the brachial valve may show the posterior part of the inner pair of the vascular ridges frequently found in species of Strophomena. The radiating striae are much coarser than in most species of Strophomena. About 7 to 9 striae occupy a width of 5 mm. near the margin of the brachial valve, while 7 to 10 occur in the same width on the pedicle valve.
- Shell moderately large, wider than long, margins narrowly rounded. Hinge line straight; cardinal extremeties subrectangular; lateral profile convexo-concave. Ventral valve with a swollen umbo, slightly concave anterolaterally. Interarea long, strongly apsacline. Pseudodeltidium large, convex. Dorsal valve dome-shaped, highest about in middle. Interarea short, anacline. Chilidium small, convex. Ornamental costellate, costellae about equal in size; growth lines strong.
- Horizon and Locality: Lower part of Elgin member; along gutter of north-south road, about 4 miles south of Wykoff, Fillmore county, Minnesota.